Use of Stable Isotopes to Trace the Fate of Applied ... · Chapter 3 results showed an increase in timing and development of foliage in fertilized compared to unfertilized plots

J.E. Raymond

Use of Stable Isotopes to Trace the Fate of Applied Nitrogen in Forest

Plantations to Evaluate Fertilizer Efficiency and Ecosystem Impacts

Jay Edwards Raymond

Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

In

Forestry

Thomas R. Fox, Chair John E. Barrett

Brian D. Strahm Valerie A. Thomas February 3rd, 2016 Blacksburg, VA

Keywords: 15N; forest fertilization; nitrogen cycle; plantation forestry

Copyright 2016, Jay Edwards Raymond

J.E. Raymond

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Use of Stable Isotopes to Trace the Fate of Applied Nitrogen in Forest Plantations to

Evaluate Fertilizer Efficiency and Ecosystem Impacts

Jay Edwards Raymond

ABSTRACT

This study assessed five fertilizer treatments (control – no fertilizer, urea, urea treated with N-(n-butyl) thiophosphoric triamide (NBPT), coated urea + NBPT (CUF), polymer coated urea (PCU) ) during two application seasons (spring, summer) to: 1) compare fertilizer nitrogen (N) losses (see Chapter 2); 2) evaluate temporal N uptake patterns of loblolly pine (see Chapter 3); and 3) evaluate fertilizer N cycling and partitioning in a loblolly pine ecosystem (see Chapter 4).

Chapter 2 results showed enhanced efficiency fertilizers (EEFs) significantly reduced ammonia (NH3) volatilization losses compared to urea. Mean NH3 volatilization after spring fertilization ranged from 4% to 26% for EEFs versus 26% to 40% for urea, and 8% to 23% for EEFs versus 29% to 49% for urea in summer. Chapter 3 results showed an increase in timing and development of foliage in fertilized compared to unfertilized plots. In addition, the cumulative N uptake by loblolly pines increased over the entire growing season from N originating from fertilizer and natural sources. Chapter 4 results showed greater fertilizer N recovery for EEFs in both spring and summer (80%, 70-80% respectively) compared to urea (60%, 50% respectively) with most fertilizer N recovered from mineral soil (20% to 50%) and loblolly pines (10% to 50%).

Three primary conclusions come from this research: 1) EEFs reduce NH3 volatilization after N fertilization compared to urea regardless of application timing and weather conditions (see Chapter 2); 2) N uptake by loblolly pines increases over the entire growing season after N fertilization (see Chapter 3); more fertilizer N remains in the ecosystem with EEFs compared to urea with most fertilizer N remaining in the soil (see Chapter 4). From these findings, we hypothesize that the EEFs in this study: 1) reduce ammonia volatilization which 2) translates to an increase in fertilizer nitrogen remaining in the loblolly pine plantation system that 3) increases the amount of plant available nitrogen for an extended period into the stand rotation and 4) increases fertilizer nitrogen use efficiency (FNUE) for all enhanced efficiency fertilizers investigated in this study compared to the conventional form of fertilizer N used in forestry, urea.

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Dedication

To my wife Anja Whittington.

Without her understanding, patience and support at every level,

this process would have been less enjoyable and less meaningful.

Thank you for everything you have done for me and for us over the last several years.

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Acknowledgements

I would like to extend my sincerest gratitude to my advisor, Dr. Thomas R. Fox, who

provided the guidance, advice, and support during this entire process. I would like to thank my

committee member Dr. Brian D. Strahm, who also provided guidance and advice during this

process. I would like to thank my committee members, Dr. John E. Barrett and Dr. Valerie A.

Thomas, for their support and guidance during the development of my dissertation research.

I would like to thank Dave Mitchem for always making time in the laboratory to assist

me. I would like to thank Eric Carbaugh, Andy Laviner, and Kevan Minick for the numerous

conversations on the meaning of science, and their friendships during this process. I would like

to thank Dr. Ivan Fernandez for his mentoring and advice during various phases of moving

through this process. I would also like to thank Dr. Anja Whittington for her input and review

during all stages of this process.

I finally would like to thank the Forest Productivity Cooperative, the Center for

Advanced Forestry Systems, and the United States Department of Agriculture National Institute

of Food and Agriculture for their support of this research.

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Contributing Authorship

Chapter 2.

Chapter 2 will be submitted to Soil Biology and Biochemistry in 2016

J.E. Raymond T.R. Fox

B.D. Strahm J.L. Zerpa

Chapter 3.

Chapter 3 will be submitted to Forest Ecology and Management in 2016



Chapter 4.

Chapter 4 will be submitted to Ecology in 2016



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Table of Contents Abstract ........................................................................................................................................... ii Dedication ...................................................................................................................................... iii Acknowledgements ........................................................................................................................ iv Contributing Authorship ..................................................................................................................v Table of Contents ........................................................................................................................... vi List of Tables ................................................................................................................................. ix List of Figures ............................................................................................................................... xii Chapter 1. Introduction

1.1. Justification ...........................................................................................................................1 1.2. Literature Review

1.2.1. The Nitrogen Cycle in Forest Ecosystems .....................................................................4 1.2.2. Productivity of Intensively Managed Pine Plantations ..................................................6 1.2.3. Assessment Methods for Nutrient Deficiencies in Loblolly Pine ..................................7 1.2.4. Nitrogen Fertilization of Pine Plantations ......................................................................8 1.2.5. Cycling of Fertilizer Nitrogen in Pine Plantations .......................................................10 1.2.6. Enhanced Efficiency Fertilizers ...................................................................................13 1.2.7. Use of Stable Isotopes to Trace Nitrogen in Forest Ecosystems ..................................16

1.3. Objectives ............................................................................................................................20 Literature Cited ..........................................................................................................................21

Chapter 2. Ammonia volatilization following nitrogen fertilization with enhanced

efficiency fertilizers and urea in loblolly pine (Pinus taeda L.) plantations of the

southern United States

Abstract ......................................................................................................................................41 2.1. Introduction .........................................................................................................................42 2.2. Materials and Methods

2.2.1. Experimental Design ....................................................................................................46 2.2.2. Fertilizer Treatments ....................................................................................................46 2.2.3. Site Description ............................................................................................................47 2.2.4. Experimental Method ...................................................................................................47 2.2.5. Laboratory Processing ..................................................................................................48 2.2.6. Weather Data ................................................................................................................49 2.2.7. Calculations ..................................................................................................................50 2.2.8. Statistical Analysis .......................................................................................................51

2.3. Results 2.3.1. Differences in NH3 Volatilization Among Fertilizers Treatments – Spring .................52 2.3.2. Differences in NH3 Volatilization Among Fertilizers Treatments – Summer .............52 2.3.3. NH3 Volatilization Among Fertilizer Treatments – Spring + Summer ........................53 2.3.4. Correlation Between NH3 Volatilization from Urea and Weather Data ......................53

2.4. Discussion ...........................................................................................................................54

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2.5. Conclusion ...........................................................................................................................62 2.6. Acknowledgements .............................................................................................................64 Literature Cited ..........................................................................................................................65

Chapter 3. Temporal patterns of foliar nitrogen after fertilization for two application

seasons in mid-rotation loblolly pine (Pinus taeda L.) plantations of the southern United

States

Abstract ......................................................................................................................................82 3.1. Introduction .........................................................................................................................83 3.2. Materials and Methods

3.2.1. Experimental Design and Site Description...................................................................86 3.2.2. Fertilizer Treatments ....................................................................................................87 3.2.3. Experimental Method – Field Sampling.......................................................................88 3.2.4. Experimental Method – Laboratory Processing ...........................................................88 3.2.5. Calculations ..................................................................................................................89 3.2.6. Statistical Analysis .......................................................................................................89

3.3. Results 3.3.1. Flush Development .......................................................................................................90 3.3.2. Individual Fascicle Mean Mass ....................................................................................91 3.3.3. Foliar N Mean Concentration .......................................................................................92 3.3.4. Individual Fascicle Mean N Content ............................................................................92 3.3.5. Foliar 15N ......................................................................................................................94 3.3.6. Individual Fascicle Fertilizer Mean N Content ............................................................94 3.3.7. Total Cumulative Individual Fascicle N Content .........................................................96 3.3.8. Total Cumulative Individual Fascicle Fertilizer N Content .........................................97

3.4. Discussion ...........................................................................................................................97 3.5. Conclusion .........................................................................................................................104 3.6. Acknowledgements ...........................................................................................................105 Literature Cited ........................................................................................................................106

Chapter 4. Understanding the fate of applied fertilizer nitrogen in mid-rotation loblolly

pine plantations (Pinus taeda L) of the southern United States using stable isotopes

Abstract ....................................................................................................................................129 4.1. Introduction .......................................................................................................................131 4.2. Materials and Methods

4.2.1. Experimental Design and Site Description.................................................................137 4.2.2. Fertilizer Treatments ..................................................................................................137 4.2.3. Experimental Method – Field Sampling – Pre-treatment ...........................................138 4.2.4. Experimental Method – Field Sampling – Post Treatment ........................................139 4.2.5. Experimental Method – Laboratory Processing .........................................................140 4.2.6. Calculation of Fertilizer Recovery .............................................................................141 4.2.7. Statistical Analysis .....................................................................................................142

4.3. Results 4.3.1. Study 1 – 2011 Application – Spring versus Summer ...............................................143 4.3.2. Study 2 – 2012 Application - Spring ..........................................................................145

4.4. Discussion .........................................................................................................................147 4.5. Conclusions .......................................................................................................................152 4.6. Acknowledgements ...........................................................................................................154

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Literature Cited ........................................................................................................................155

Chapter 5. Conclusions

5.1. Introduction .......................................................................................................................182 5.2. Chapter Results and Discussion ........................................................................................185 Literature Cited ........................................................................................................................192

Appendix ......................................................................................................................................194

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List of Tables Table 1.1. Common types of nitrogen containing fertilizers for use in agriculture, horticulture and turf grass management. Top portion of list has general applications to forestry. Information summarized from tables specific to nitrogen containing fertilizers in Havlin et al. (2014), IPNI (2015), PSU (2015) ..................................................................................................38 Table 1.2. Common types of enhanced efficiency fertilizer products for use with nitrogen containing fertilizers in agriculture, horticulture, and turf grass management. Certain products have current applications to forestry. Information summarized from tables specific to slow and controlled release fertilizers found in Hauck (1985), Shaviv (1996), Trenkel (2010), Azeem et al. (2014), Havlin et al. (2014), IPNI (2015) and PSU (2015). Primary resource Tabe 4-23 in Havlin et al. (2014) ....................................................................................39 Table 2.1. Selected climatic and physical characteristics for sites in the southern United States selected to evaluate ammonia volatilization following fertilization with urea and enhanced efficiency fertilizers enriched with 15N ..........................................................................................74 Table 2.2. Selected climatic and physical characteristics for sites in the southern United States selected to evaluate ammonia volatilization following fertilization with urea and enhanced efficiency fertilizers enriched with 15N ..........................................................................................75 Table 2.3. Selected physical and chemical soil characteristics of loblolly pine stands in the southern United States selected to evaluate ammonia volatilization following fertilization with urea and enhanced efficiency nitrogen fertilizers ..................................................................76 Table 2.4. Mean 15N (‰) values for depth increments of microcosms 30 days after a spring and summer application of δ15N enriched fertilizer treatments (CUF, NBPT, PCU, urea) at 6 mid-rotation loblolly pine plantations across the southern United States. The nitrogen application rate was 224 kg N ha-1. Values in parentheses are the standard error of the mean .....78 Table 2.5. Results (F-values and significance) of repeated measures ANOVA testing effects of treatment (CUF, NBPT, PCU, urea), season (spring, summer), day (1, 15, 30), and the interactions of treatment*season, treatment*day, and treatment*season*day for ammonia volatilization loss as a percentage of applied 15N enriched fertilizer for sites at 6 mid-rotation loblolly pine plantations across the southern United States. The nitrogen application rate was 224 kg N ha-1 ..................................................................................................................................79 Table 2.6. P > | t | values for treatment (CUF, NBPT, PCU, urea) contrasts for mean ammonia volatilization loss for days (1, 15, 30) after application of 15N enriched fertilizers during two seasons (spring, summer) and combined (spring + summer) at 6 midrotation loblolly pine plantations in the southern United States. The nitrogen application rate was 224 kg N ha-1 ........80 Table 2.7. Correlation matrix for selected weather variables 1 day following urea application for cumulative mean volatilization loss after application of 15N enriched nitrogen fertilizers during two seasons (spring, summer) and combined seasons (spring + summer) at 6 mid-rotation loblolly pine plantations in the southern United States. Top values are the Pearson Correlation and number below in parentheses represents the associated p-value. The nitrogen application rate was 224 kg N ha-1 .................................................................................................81

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Table 3.1. Selected climate and physical characteristics of loblolly pine stands in the southern United States selected to evaluate temporal foliar uptake of fertilizer nitrogen following the application of urea or enhanced efficiency nitrogen fertilizers enriched with 15N ......................121 Table 3.2. Selected characteristics of loblolly pine stands in the southern United States selected to evaluate selected to evaluate temporal foliar uptake of fertilizer nitrogen following the application of urea or enhanced efficiency nitrogen fertilizers enriched with 15N. ...............122 Table 3.3. The number of study sites where flushes were observed for each treatment (control, CUF, NBPT, PCU, urea) in sampling periods after a spring and summer fertilization for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers enriched with 15N. (*) represents date of fertilization. (-) represents potential flushes not measured in the sampling period .................123 Table 3.4. The mean individual fascicle mass (g) of flushes in sampling periods after a spring and summer fertilization for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers enriched with 15N. Different letters represent significant differences at α = 0.05 among treatments in a sampling period. Numbers in parentheses are the standard error of the mean, and absent values and letters indicate post-hoc analysis not conducted due to a limited sample size. The N application rate was 224 kg N ha-1. N = 6. ...............................................................................124 Table 3.5. The mean foliar N concentration (%) of individual flushes for sampling periods after a spring and summer fertilization for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers enriched with 15N. Different letters represent significant differences at α = 0.05 among treatments in a sampling period. Numbers in parentheses are the standard error of the mean, and absent values and letters indicate post-hoc analysis not conducted due to a limited sample size. The N application rate was 224 kg N ha-1. N = 6 ................................................................125 Table 3.6. The mean individual fascicle N content (mg) of flushes per sampling periods after a spring and summer fertilization for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers enriched with 15N. Different letters represent significant differences at α = 0.05 among treatments in a sampling period. Numbers in parentheses are the standard error of the mean, and absent values and letters indicate post-hoc analysis not conducted due to a limited sample size. The N application rate was 224 kg N ha-1. N = 6 ................................................................126 Table 3.7. The mean foliar 15N (‰) of individual flushes for sampling periods after a spring and summer fertilization for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers enriched with 15N. Different letters represent significant differences at α = 0.05 among treatments in a sampling period. Numbers in parentheses are the standard error of the mean, and absent values and letters indicate post-hoc analysis not conducted due to a limited sample size. The N application rate was 224 kg N ha-1. N = 6 ................................................................................127

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Table 3.8. The mean individual fascicle fertilizer N (mg) content for flushes in sampling periods of loblolly pine stands in the southern United States selected to evaluate seasonal fertilizer N uptake of urea or enhanced efficiency N containing fertilizers enriched with 15N after a spring and summer fertilization. Different letters represent significant differences at α = 0.05 among treatments in a sampling period. Numbers in parentheses are the standard error of the mean, and absent values and letters indicate post-hoc analysis not conducted due to a limited sample size. The N application rate was 224 kg N ha-1. N = 6 .......................................128 Table 4.1. Selected climate and physical characteristics of loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer N following application of urea or enhanced efficiency N fertilizers enriched with 15N. MAP = Mean annual precipitation. MAT = Mean annual temperature .........................................................................168 Table 4.2. Characteristics of loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer N following application of urea or enhanced efficiency N fertilizers enriched with 15N ....................................................................................170 Table 4.3. Selected physical and chemical soil characteristics of loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer N following application of urea or enhanced efficiency N fertilizers enriched with 15N ................................171 Table 4.4. The mean 15N (‰), N (g kg-1) and fertilizer N recovery (% of fertilizer N applied) for individual ecosystem components for Study 1 (spring + summer 2011) for loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer N for urea or enhanced efficiency N containing fertilizers enriched with 15N. Different letters represent significant differences at α = 0.05. Numbers in parentheses represent the standard error of the mean. The N application rate was 224 kg N ha-1. N = 10 .........................................173 Table 4.5. Analysis of variance for treatment differences for fertilizer N recovery (% of fertilizer N applied) for total and the primary ecosystem components for Study 1 (spring + summer 2011) at mid-rotation loblolly pine plantations in the southern United States. N application rate was 224 kg N ha-1. N = 10 .................................................................................175 Table 4.6. The mean 15N (‰), N (g kg-1) and fertilizer N recovery (% of applied fertilizer N) for individual ecosystem components for Study 2 (March-April 2012)for loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer N for urea or enhanced efficiency N containing fertilizers enriched with 15N. Different letters represent significant differences at α = 0.05. Numbers in parentheses represent the standard error of the mean. The N application rate was 224 kg N ha-1. N = 12 for all ecosystem components except sapling/pole and shrub strata where N = 6 ...................................................177 Table 4.7. Analysis of variance for treatment differences for fertilizer N recovery (% of fertilizer N applied) for total and the primary ecosystem components for Study 2 (spring 2012) at mid-rotation loblolly pine plantations in the southern United States. N application rate was 224 kg N ha-1. N = 12 except for Understory where N = 7 ...........................................179 Table 4.8. P > | t | values for fertilizer N treatment (CUF, NBPT, PCU, urea) contrasts for total ecosystem fertilizer N recovery (% of fertilizer N applied) for Study 1 (spring + summer 2011) and Study 2 (spring 2012) at mid-rotation loblolly pine plantations in the southern United States. The N application rate was 224 kg N ha-1. N =10 ................................................181

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List of Figures

Figure 1.1. The nitrogen cycle in forested ecosystems. Adapted from Fisher and Binkley (2000), Dawson et al. (2002), Sylvia et al. (2005), Groffman and Rosi-Marshall (2013) .........................35 Figure 1.2. Soil nitrogen supply and tree nitrogen demand during a typical rotation for loblolly pine in the southern United States. Adapted from Allen (1990) and Fox et al. (2007a) ...............36 Figure 1.3. The fertilizer nitrogen cycle in a coniferous forest. Adapted from Crane (1972) .......37 Figure 2.1. Location map for sites selected in the southern United States to evaluate ammonia volatilization following fertilization with urea and enhanced efficiency fertilizers enriched with 15N after a spring and summer fertilizer application ......................................................................70 Figure 2.2. Microcosm diagram for ammonia volatilization study in the southern United States selected to evaluate ammonia volatilization following fertilization with urea and enhanced efficiency fertilizers enriched with 15N after a spring and summer application ............................71 Figure 2.3. The mean percent recovery of applied 15N enriched fertilizers in microcosms, by soil depth increment, for spring and summer application after 30 days. The >30 cm increment is immediately below the microcosm. Error bars are the standard error of the mean .......................72 Figure 2.4. Cumulative mean ammonia volatilization loss from microcosms, expressed as a percent loss from nitrogen fertilizer treatments, for 6 mid-rotation loblolly pine plantations across the southern United States after a spring (A), summer (B) and spring + summer (C) application of 15N enriched fertilizer treatments (CUF, NBPT, PCU, urea). The nitrogen application rate was 224 kg N ha-1. Error bars are the standard error of the mean.......................73 Figure 3.1. Location map of loblolly pine stands in the southern United States selected to evaluate temporal foliar uptake of fertilizer nitrogen during the growing season using 15N enriched fertilizers following the application of urea or enhanced efficiency nitrogen fertilizers after a spring and summer fertilizer application ..........................................................................114 Figure 3.2. Representation of individual flush growth on a loblolly pine sampled every six weeks over the growing season after a spring fertilization in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers. .................115 Figure 3.3. Cumulative individual fascicle N content (mg/fascicle) of individual flushes combined for sampling weeks (6, 12, 18, 24, 30) after spring and summer fertilization (2011) for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers enriched with 15N. The N application rate was 224 kg N ha-1. N = 6. ...................................................................................................................116

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Figure 3.4. Cumulative individual fascicle N content from fertilizer (mg) of individual flushes (0, 1, 2) combined for sampling weeks (6, 12, 18) after spring and summer fertilization (2011) for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers enriched with 15N. The N application rate was 224 kg N ha-1. N = 6. ...................................................................................................................117 Figure 4.1. Location map of loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer N following the application of urea or enhanced efficiency N fertilizers enriched with 15N ....................................................................................164 Figure 4.2. Plot sampling schematic for loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer N following application of urea or enhanced efficiency N fertilizers enriched with 15N. Boxes below each category (Crop Tree, Understory, Soils, Litterfall) represent portions of the ecosystem sampled ................................165 Figure 4.3. The total fertilizer N recovery (% of fertilizer N applied) of the major ecosystem components (tree - loblolly pine aboveground + belowground biomass, litterfall, and soil (O horizon + mineral soil- 0 to 30cm)) for loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer N of urea or enhanced efficiency N fertilizers enriched with 15N. Data represents combined fertilizer application dates for Study 1 for spring (March-April 2011) and summer (June 2011). Study 1 did not have a significant Understory category as in Study 2. Different letters represent significant differences at α = 0.05. The N application rate was 224 kg N ha-1. N =10 ..................................................................................166 Figure 4.4. The total fertilizer N recovery (% of fertilizer N applied) of the major ecosystem components (Tree - loblolly pine aboveground + belowground biomass, Understory, Litterfall, and Soil (O horizon + mineral soil- 0 to 30cm)) for loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer N of urea or enhanced efficiency N fertilizers enriched with 15N. Data represents fertilizer application dates for Study 2 for spring (March-April 2012). Different letters represent significant differences at α = 0.05. The N application rate was 224 kg N ha-1. N = 12 .................................................................................167

Figure 5.1. Implication and directions of current and future research for improving the understanding of nitrogen cycling after fertilization with N containing fertilizers in mid-rotation loblolly pine plantations across the southern United States.........................................................191

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Chapter 1. Introduction

1.1. Justification

Fertilizers, primarily containing nitrogen (N) and phosphorous (P), have been used for

several decades in loblolly pine (Pinus taeda L.) plantations in the southern United States to

improve productivity by ameliorating widespread nutrient deficiencies of N and P occurring in

these southern forest soils (Allen 1987, Fischer and Binkley 2000, Richter et al. 2000, Allen et

al. 2005, Fox et al. 2007a, b). Consequently, over 500,000 ha of forested plantations are fertilized

annually with N and P in the southern United States (Albaugh et al. 2004). Other nutrients, such

as potassium (K) and/or micronutrients, may also be limiting or co-limiting but are applied on a

site specific basis when these deficiencies are observed (Albaugh et al. 1998, 2004, Sampson and

Allen 1999, Jokela and Martin 2000, Vogel and Jokela 2011, Carlson et al. 2014).

Despite significant gains in forest productivity from fertilization, a large degree of

uncertainty exists in the fate of the fertilizer N in these plantation systems (Fox et al. 2007a). The

majority of fertilizer N applied in southern pine plantations is urea (CO(NH2)2) (Allen 1987).

Urea is the preferred source of N because of a high N content (46%) and overall low cost per unit

(Allen 1987, Fox et al. 2007b). However, potentially large losses of fertilizer N can occur

following fertilization with urea due to ammonia (NH3) volatilization, with losses generally

correlated with weather conditions at the time of application (Kissel et al. 2009, Zerpa and Fox

2011, Elliot and Fox 2014). Losses due to NH3 volatilization in acidic forest soils are highly

variable and range from 5% to 90% (Boomsma and Pritchett 1979, Craig and Wollum 1982,

Kissel et al. 2004, Kissel et al. 2009, Zerpa and Fox 2011, Elliot and Fox 2014).

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The amount of urea N lost from the system by NH3 volatilization decreases the amount of

fertilizer N potentially available to crop trees (loblolly pines) and likely contributes to low rates

of fertilizer N uptake (Elliot and Fox 2014). Additional loss pathways of fertilizer N include

denitrification (Shrestha et al. 2014) or leaching (Vitousek and Matson 1985a, Binkley et al.

1999, Meason et al. 2004, Aust and Blinn 2004). Although field experiments show crop tree

fertlizer N uptake ranges from 25% to 50% (Baker et al. 1974, Mead and Pritchett 1975,

Heilman et al. 1990, Johnson and Todd 1988, Li et al. 1991, Albaugh et al. 1998, 2004, Blazier

et al. 2006), laboratory pot studies show plant uptake of fertilizer N ranges from 50% to near

100% in lodgepole pine (Pinus contorta Dougl. ex. Loud.) (Amponsah et al. 2004), Douglas-fir

(Pseudotsuga menziesii (Mirb.) Franco) (Pang 1985), and black walnut (Juglans nigra L.) (Salifu

et al. 2009). Conversely, in a 3 year old loblolly pine plantation in Oklahoma fertilized with urea

and diammonium phosphate, fertilizer N recovery ranging from 6% to 25% (Blaizer et al. 2006).

Mead et al. (2008) found on average 14.5% of fertilizer N was recovered 10 years after

application in a 38 to 39 year old Douglas-fir stand in British Columbia. Heilman at al. (1990)

found higher N recoveries, averaging 30% in 7 to 9 year old Douglas-fir forests in Washington.

Unfortunately, none of these represent southeastern loblolly pine mid-rotation fertilization

conditions.

The remaining fertilizer N not lost from the system or taken up by the crop trees is

partitioned among other ecosystem components including the understory vegetation (competing

volunteers, deciduous trees, shrubs, vines, herbaceous species), litter, forest floor (organic

horizon), and mineral soil (Birk and Vitousek 1986, Albaugh et al. 2004). The cumulative effect

of reduced fertilizer N uptake by desired crop trees translates to lower fertilizer nitrogen use

efficiency (FNUE), defined as the ratio between the amount of fertilizer N removed with a crop

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and the amount of fertilizer N applied (Havlin et al. 2014). An improved understanding of the

proportion of fertilizer N lost from the system, incorporated by crop trees and partitioned in

ecosystem components is hindered by our inability to quantify the cycling of fertilizer N and its

ultimate fate in forest ecosystems.

Although many loblolly pine plantations respond positively to the application of N and/or

other nutrients, some do not (Pritchett and Smith 1975, Martin et al. 1999, Amateis et al. 2000,

Carlson et al. 2014). Some stands may not respond to N additions because large amounts are lost

from the system, immobilized in the soil by microbial communities or on exchange sites, or the

site may not be N deficient due to high rates of mineralization from decomposing organic matter.

Additionally, stands may not respond because deficiencies of elements other than N create

nutrient imbalances that limit growth (Vogel and Jokela 2011, Carlson et al. 2014) and would

respond to fertilization if the required nutrients could be properly identified and effectively

applied.

The inability to accurately determine sites specific responsive to fertilization influences

forest management decisions. Until our understanding of the mechanisms and interactions

governing the movement of fertilizer N through loblolly pine plantations on a site-specific basis

improves, the ability to accurately predict response will be limited. The goal of this research is to

enhance our fundamental understanding of the fate of fertilizer N in loblolly pine plantations to

improve the economic feasibility of fertilization and reduce potential negative environmental

impacts of fertilizer N loss from the system. This research used N containing fertilizers enriched

with the stable isotope 15N in a series of pulse-chase experiments. We compared the fate and

cycling of conventional (urea) and enhanced efficiency N containing fertilizers (coated Urea +

NBPT (CUF), urea + NBPT (N-(n-Butyl) thiophosphoric triamide) (NBPT), polymer coated urea

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(PCU)) in loblolly pine plantations. This research will help determine the feasibility of using

enhanced efficiency N containing fertilizers to improve fertilizer nitrogen use efficiency.

1.2. Literature Review

1.2.1. The Nitrogen Cycle in Forest Ecosystems

Nitrogen is generally the most limiting element to primary productivity in terrestrial

ecosystems because it is an essential component of key molecules such as deoxyribonucleic acid

(DNA), ribonucleic acid (RNA), amino acids, proteins, enzymes that facilitate biochemical

reactions, and photosynthesis (Miller 1981, Chapin et al. 1986, Vitousek and Howarth 1991,

Lambers et al. 2005). Reduced N availability to trees restricts leaf area production, and forest

plantations with low leaf area do not achieve optimal growth (Linder 1987).

Although terrestrial systems contain large quantities of N, only a small fraction is

generally plant available at any given time (Likens and Bormann 1995, Fisher and Binkley

2000). While the atmosphere contains the largest pool of N (~79%), it exists as unreactive

dinitrogen gas (N2) and is not available for uptake by vascular plants. The strong triple bonds of

N2 must be broken and N must then be synthesized into plant available forms in biochemical

reactions which include ammonium (NH4+), ammonia (NH3), volatile organic compounds

(VOCs), nitrate (NO3-), organic forms of nitrogen (e.g. urea), or from fertilizer production

(Galloway et al. 2004, Lambers et al. 2005). In most forests, the primary N cycling pathways are

biological fixation, mineralization, immobilization, nitrification, and denitrification (Fisher and

Binkley 2000) (Figure 1.1).

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Bonds of atmospheric N2 can be broken by lightning, fertilizer production, nonsymbiotic

(free living microbes) or symbiotic (microbes inside roots nodules of plants) microbial mediated

dinitrogen fixation (nitrogen fixation) (Sylvia et al. 2005). Nitrogen fixation is the conversion of

N2 to ammonia (NH3). Nonsymbiotic N fixation is low for most terrestrial ecosystems and

confined to substrates high in energy but low in N, such as coarse woody debris (Groffman and

Rosi-Marshall 2013). Conversely, symbiotic N fixation can be high where the symbiosis between

plant and N fixing soil microbes provides a competitive advantage (Vitousek et al. 2002,

Groffman and Rosi-Marshall 2013). Nitrogen fixation may decline with ecosystem succession

but can still be important for N inputs to the system over extended periods (Grant and Binkley

1987, Vitousek et al. 2002).

Nitrogen mineralization is the transformation of organic N to NH4+, and immobilization

is the incorporation of assimilation of NH4+ or NO3

- into organic components. These processes

are considered the primary sources of plant available N in most forested ecosystems (Miller et al.

1979, Likens and Bormann 1995, Groffman and Rosi-Marshall 2013). Nitrogen mineralization is

partly controlled by soil microbial requirements for carbon (C) and N during substrate utilization,

where C and N are incorporated into microbial biomass or lost through respiration. Adequate N

in substrates used by soil microbes will generally translate to an accumulation of N in the soil

that increases plant available N (Vitousek and Matson 1984, 1985b, Hart et al. 1994, Scott and

Binkley 1997, Finzi et al. 1998, Piatek and Allen 1999, Vitousek et al. 2002). Yet if N is limiting

in substrates utilized by soil microbes, N will be removed from mineral soil pools to satisfy

microbial demand and immobilized, reducing plant available N (Vitousek and Matson 1985b,

Davidson et al. 1990, Groffman et al. 1993, Scott and Binkley 1997).

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Nitrification, the conversion of NH4+ to nitrite (NO2

-) then nitrate (NO3-), is limited by

soil NH4+ concentrations, because nitrifying microbes (bacteria and archaea) are generally poor

competitors for NH4+ compared to heterotrophic microbes (Ward 2011, Norman and Barrett

2014, Norman et al. 2015). During nitrification N may be lost from the system due to high NO3-

mobility (leaching). Nitrification is generally greater in systems with high N availability such as

annually fertilized agriculture fields or recently disturbed ecosystems (Groffman and Rosi-

Marshall 2013). Although NO3- may be lost from upland systems by nitrification, it can be

transformed to N2 or other gaseous N compounds by denitrification in anaerobic environments

(Schimel and Bennett 2004, Ward 2011).

Denitrification is a form of anaerobic respiration utilizing N compounds along a redox

gradient and sequence of electron acceptors that produce reduced products (Sylvia et al. 2005).

Complete denitrification is the conversion of NO3- to N2 with intermediary compounds of NO2

-,

nitric oxide (NO) and nitrous oxide (N2O) (Sylvia et al. 2005). Dependent on the environmental

conditions and microbial communities, incomplete denitrification may occur with intermediary

compounds released from the system (Vitousek and Matson 1984, 1985b, Vitousek and Howarth

1991).

1.2.2. Productivity of Intensively Managed Pine Plantations

Forests provide numerous functions and services to society including clean water and air,

carbon sequestration, biodiversity, food for human consumption, commodities for economic

development, and diverse recreational opportunities (Nabuurs et al. 2007). Wood produced in

forests is a primary economic commodity serving as a raw material for industries, especially in

the southern United States (Howard 2003). Demands for both traditional forest products, such as

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pulp and timber, and new products for emerging markets such as the bioenergy industry, are

increasing (Howard 2003). How to balance increasing demand for wood with non-commodity

services and values produced by forests is a key question facing modern society (Fox 2000).

Most forests in the United States regenerate naturally, are extensively managed, and have

low productivity (Fox et al. 2007b). The productivity of these extensively managed forests is not

sufficient to produce the raw resources required to supply competing societal demands for forest

products, and larger areas of natural forests will be required to meet commodity demands (Sedjo

2001). Yet high growth rates of intensively managed tree plantations can supply large quantities

of wood and assist in meeting increasing demand for raw materials (Sedjo 2001, WWF 2015).

In the United States, plantation forests are primarily concentrated in the South (Oswalt et

al. 2014) occupying 12.8 million ha of mostly loblolly pine (Albaugh et al. 2007). Intensive

silvicultural management can significantly increase the productivity of these forests, from less

than 2 m3 ha-1 yr-1 to more than 10 m3 ha-1 yr-1 (Fox et al. 2007b). Although improved growth

rates have been achieved by intensive silviculture management, theoretical models and empirical

field trials indicate productivity of 20 m3 ha-1 yr-1 for 15 year rotations are possible in these

southern pine plantations (Allen et al. 2005, Fox et al. 2007b). Increases in future productivity

must include site-specific silvicultural prescriptions to ameliorate growth limiting factors in an

economically viable and environmentally responsible manner.

1.2.3. Assessment methods for nutrient deficiencies in loblolly pines

Numerous methods have been developed to assess loblolly pine nutrient status and

evaluate potential response to fertilization (Bowen and Nambier 1984, Carter 1992). The

simplest method is visual observation (Stone 1968) but is of limited value because obvious

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symptoms (chlorosis) can be due to multi-nutrient deficiencies (Vogel or Jokela 2011) or

pathogens (USDA 1989). Foliar analysis including nutrient concentrations, critical nutrient levels

(Tisdale et al. 1985, Havlin et al. 2014), nutrient ratios in Diagnosis of Recommendation

Integrated System (DRIS) (Hockman and Allen 1990, Sypert 2005), and graphical analysis

(Haase and Rose 1985) improve on visual assessment, but also may be of limited value if

sampling season and edaphic factors are not integrated (Carter 1992). Soil sampling and nutrient

addition experiments (Binkely and Hart 1989) also provide insight but may require large sample

sizes to address soil heterogeneity. Integrated soils research has led to the development of soil

groupings based on similar parent materials within physiographic regions that correlate well with

specific nutrient deficiencies, like the Cooperative Research in Forest Fertilization (CRIFF) soil

groupings in the southern United States (Fisher and Garbett 1980, Jokela and Long 2000,

Villanueva 2015), or groupings in the Pacific Northwest (Littke et al. 2014). Field or greenhouse

bioassays using seedlings can also be valuable, but results may be confounded by artificial

laboratory conditions and may be difficult to extrapolate to the field (Addoms 1937, Hobbs 1947,

Morrison 1974, Mead and Pritchett 1975, Birk and Vitousek 1986). The most reliable, but

expensive, assessment of nutrient deficiencies and fertilizer N response are fertilizer field trials

(Miller 1981, Carter 1992, Albaugh et al. 2004, Fox et al. 2007a).

1.2.4. Nitrogen Fertilization of Pine Plantations

Low availability of plant available soil nutrients, primarily N (Albaugh et al. 1998,

Sampson and Allen 1999, Jokela and Martin 2000, Fox et al. 2007b) and P (Pritchett and

Comerford 1982, Gent et al. 1986, Fox et al. 2007b) reduce productivity of many pine

plantations across the southern United States. Low nutrient availability reduces leaf area

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production and decreases photosynthetic capacity, and stands with suboptimal leaf area do not

grow at their maximum potential (Linder 1987, Fox et al. 2007b). Fertilization of these systems

can increase soil nutrient availability, translating to increased leaf area and hence improved stand

productivity. Empirical results from fertilization field trials in intensively managed loblolly pine

plantations show most nutrient limitations can be ameliorated with fertilization (Fox et al.

2007a).

Nutrient limitations for plants in most terrestrial ecosystems develop when the nutrient

demand exceeds the nutrient supply (Miller 1981, Chapin et al. 1986, Allen et al. 1990). In forest

plantations, stand N uptake progresses in a sigmoidal pattern, increasing with stand development as

plant N demand becomes unbalanced with plant N availability (Figure 1.2) (Switzer and Nelson

1972, Wells and Jorgenson 1975). Plant N availability in the soil can increase after site disturbances,

such as forest harvesting because conditions (temperature, moisture, aeration) favor decomposition

of the forest floor, coarse woody debris, and root turnover (Vitousek and Matson 1985, Fox et al.

1986). As the stand age progresses, N is increasingly immobilized in pools with varying turnover

times, and the environmental conditions conducive to high N availability decrease. The plant

available N required to maintain maximum growth gradually shifts until plant demand for N

exceeds the supply of plant available N in the soil (Miller 1981). As the stand transitions to plant

available N limitation, N fertilization is required to maintain optimal tree growth (Figure 1.2).

Fertilization studies using 224 kg ha-1 N plus 30 kg ha-1 P on loblolly pine plantations in

the southern United States show a mean fertilization growth response of 3 m3 ha-1 yr-1 over 8

years (Fox et al. 2007a) which has led to the fertilization of over 500,000 ha of these systems

annually (Albaugh et al. 2007). Despite a positive response for most pine plantations to N + P

fertilization in the South, certain sites fail to respond or respond negatively (Pritchett and Smith

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1975, Martin et al. 1999, Amateis et al. 2000, Carlson et al 2014). These inconsistent fertilization

results in southern pine plantations have led to a range in growth of 0 m3 ha-1 yr-1 to 10 m3 ha-1 yr-1

(Fox et al. 2007b).

Several empirical field trials indicate less than 50% of fertilizer N is taken up by loblolly

pine trees (Mead and Pritchett 1975, Albaugh et al. 1998, Blazier et al. 2006). The remaining

fertilizer N is partitioned into other ecosystem components with varying turnover timescales that

include: 1) loss from the system (ammonia (NH3) volatilization, leaching, nitrification,

denitrification); 2) understory (volunteer loblolly pine, deciduous tree/shrub/vine competition,

herbaceous species); 3) litterfall; 4) forest floor; 5) soil microbial community; and/or 6) mineral

soil (Figure 1.1 and 1.3) (Birk and Vitousek 1986, Albaugh et al. 2004). The fertilizer N

remaining in the system may become available to the loblolly pines in the future (Meason et al.

2004), but uncertainty remains concerning mechanisms controlling fertilizer N turnover from

ecosystem components. The high variability and uncertainty involved with understanding crop N

tree uptake, losses, pathways and mechanisms of fertilizer N cycling may contribute to the

variability in growth response observed in fertilizer studies.

1.2.5. Cycling of Fertilizer Nitrogen in Pine Plantations

The most common fertilizer N source used in southern loblolly pine plantations is

pelletized urea (CO(NH2)2) which is surface applied via aerial or ground broadcast methods

(Allen 1987). Urea is preferred because of a high N content (46% N) and ease of transport-

storage-application, translating to the lowest overall cost per pound of any type of fertilizer N

(Gould et al. 1986, Allen 1987, Harre and Bridges 1988, Fox et al. 2007b). Other fertilizer N

forms besides urea may be used, but their use is dependent primarily on associated costs

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(storage, delivery, application, etc.) and regional availability (Havlin et al. 2014, IPNI 2015, PSU

2015) (Table 1.1). Yet fertilizer N cycling in loblolly pine plantations is uncertain, and large

losses can occur following urea application which translate to reduced fertilizer nitrogen use

efficiency (Figure 1.3).

In the acidic forest soils supporting southern loblolly pine plantations, a urea granule

undergoes a series of chemical reactions (Hauck and Stephenson 1965). Urea hydrolysis is the

initial chemical reaction and is facilitated by the extracellular enzyme urease that is prevalent in

forest soil (Conrad 1942, Pettit et al. 1976, Marsh et al. 2005). This initial hydrolysis reaction

produces ammonium carbonate that dissociates to ammonium (NH4+) and bicarbonate (HCO3

-).

The bicarbonate consumes hydrogen (H+) ions near the dissolving urea granule which raises the

surrounding pH. The ammonium ions (NH4+) can be converted to NH3 from the elevated pH near

the urea granule and lost to the atmosphere by NH3 volatilization (Koelliker and Kissel 1988).

Urea N losses via NH3 volatilization in pine plantations are highly variable, but can be

rapid and significant (Nômmik 1973, Kissel et al. 2004, Cabrera et al. 2010, Zerpa and Fox 2011,

Elliot and Fox 2014). Losses from NH3 volatilization after urea fertilization range from less than

10% (Boomsma and Pritchett 1979, Craig and Wollum 1982), 10% to 40% (Nômmik 1973,

Zerpa and Fox 2011), to greater than 50% (Kissel et al. 2004, Kissel et al. 2009, Elliot and Fox

2014). Although factors influencing NH3 volatilization following urea fertilization are well

researched in agriculture (Volk 1970, Black et al. 1987, Kissel et al. 1988), less research has

focused on forests (Volk 1970, Nômmick 1973, Kissel et al. 2004, Elliot and Fox 2014). Results

from studies primarily in agroecosystems on NH3 volatilization losses are correlated with soil pH

(Ernst and Massey 1960, Cabrera et al. 1991, Kissel et al. 2009), soil moisture (Clay et al. 1990,

Kissel et al. 2004), type of mineral soil substrate (Cabrera et al. 2005, Kissel et al. 2009, Zerpa

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and Fox 2011), relative humidity (Cabrera et al. 2005), soil temperature (Ernst and Massey 1960,

Clay et al. 1990, Moyo et al. 1989), surficial wind speed (Watkins et al. 1972, Kissel et al. 2004),

precipitation (Craig and Wollum 1982, Kissel et al. 2004) and air temperature (Gould et al. 1973,

Craig and Wollum 1982, Koelliker and Kissel 1988). A forest floors (organic horizon), present in

forests but not agriculture systems, can also effect NH3 volatilization (Cabrera et al. 2005, Kissel

et al. 2009, Zerpa and Fox 2011) because urea N after dissolution may enter voids in

decomposing pine needles and be volatilized as environmental conditions change (Cabrera et al.

2005).

Urea N lost from after fertilization from NH3 volatilization is difficult to quantify because

fertilizer N losses are caused by the interaction among many factors. Because larger NH3

volatilization losses generally occur with higher temperatures and relative humidity (Craig and

Wollum 1982, Ferguson and Kissel 1986, Moyo et al. 1989), urea is usually applied during the

winter in the South under cooler, wetter conditions to reduce the likelihood of high NH3

volatilization losses (Ferguson and Kissel 1986, Moyo et al. 1989, Cabrera et al. 2010). Under

these conditions, urea dissolution and movement of fertilizer N into the soil increases (Black et

al. 1987, Paramasivan and Alva 1997). Large urea losses via NH3 volatilization have occurred

under even under low temperatures (Carmona and Byrnes 1990, Engel et al. 2011). For example,

Kissel et al. (2004) observed low NH3 volatilization in August when NH3 volatilization is

typically high because urea was applied on the same day as a significant precipitation event, but

high NH3 volatilization (45% to 58%) was observed during the same period under simulated,

minor precipitation events. These highly variable results for NH3 volatilization after urea

fertilization highlight the difficulty in predicting the magnitude of urea N loss from NH3

volatilization after N fertilization at any time of the year.

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When environmental conditions are not conducive to NH3 volatilization after urea

dissolution, NH4+ enters the soil which is enhanced by precipitation. The NH4

+ that moves into

the soil is by soils with a high H+ buffering capacity (high organic matter, clay, silt) (Fenn and

Kissel 1976, Ferguson et al. 1984). Once in the soil, NH4+ may: 1) undergo nitrification to form

nitrate (NO3-); 2) become immobilized by soil microbes; 3) taken up by plants (loblolly pines,

understory competition); or 4) be leached from the surface soil (Figure 1.1 and 1.3). Because

most pine plantations are N deficient and fertilized intermittently, fertilizer N cycling is tightly

coupled in processes 1 to 3 with 4 (leaching) a minor long term loss (Binkely et al. 1999).

1.2.6. Enhanced Efficiency Fertilizers

Urea applied during cooler, wetter months in pine plantations of the southern United

States reduces the potential for NH3 volatilization (Allen 1987, Allen et al. 2005, Fox et al.

2007a). During winter, lower mean temperatures and higher mean precipitation increases the

probability of urea dissolution and movement into the soil, hence reducing NH3 volatilization

loss. Yet conditions following winter urea fertilization in the South can still favor high rates of

NH3 volatilization (high relative humidity, temperatures, wind speeds, low precipitation).

Additionally, winter urea application may be asynchronous to seasonal plant N demand,

decreasing FNUE (Blazier et al. 2006). One alternative to urea application in cooler, wetter

conditions to minimize NH3 volatilization is the use of slow or controlled release fertilizers,

termed enhanced efficiency fertilizers (EEFs) (Hauck 1995, Azeem et al. 2014, IPNI 2015).

To improve FNUE in agroecosystems, slow release (SRN), controlled release (CRN) and

stabilized (SNF) N fertilizers were developed (Azeem et al. 2014, Fertilizer Institute 2015). CRN

is fertilizer N that is coated or encapsulated to alter the rate, pattern and duration of N release

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(Chien et al. 2009). SRN are N fertilizer formulations that gradually release fertilizer N slowly as

a result of microbial decomposition (Trenkel 1997). SNF is fertilizer N that is treated with

chemical compounds that inhibit rapid N transformation to less stable forms (Shaviv 1996,

2005). The SRN, CRN and SNF products can all be broadly categorized as enhanced efficiency

fertilizers (EEFs) (AAPFCO 2015). The Fertilizer Institute (2015) defines EEFs as fertilizer

products that reduce nutrient loss to the environment while increasing plant nutrient availability

by slowing nutrient release or conversion of the nutrient to forms less susceptible to losses. The

EEF N containing fertilizers are designed to: 1) increase N uptake by the crop; 2) decrease N

leaching; 3) lower toxicity; 4) provide extended N supply; 5) reduce NH3 volatilization and/or

nitrification losses of N; and 6) lower application cost (Allen 1984, Hauck 1985). Therefore,

EEFs can reduce fertilizer N loss and provide greater flexibility for fertilization under a variety

of weather conditions that enhance fertilizer N uptake by the crop (Hauck 1985, Goertz 1993,

Azeem et al. 2014). Although EEFs were developed to address FNUE issues in agroecosystems,

similar issues related to FNUE exist for southern pine plantations. Implementing EEF technology

into forest fertilization programs in southern pine plantations may improve FNUE and increase

the productivity of these systems. Table 1.2 highlights several EEFs used primarily in turf grass

management, horticulture, and high value agricultural products and was primarily adapted from

Havlin et al. (2014). If manufacturing costs of these products decreases in the future, many may

become more applicable to large scale agricultural and forestry (Trenkel 2010).

One SNF mechanism to reduce urea N loss is to add urease inhibitors to urea prior to

fertilization. Urease inhibitors are substances which temporarily inhibit the hydrolytic action on

urea by the urease enzyme, resulting in less urea N lost through NH3 volatilization (AAPFCO

2015). These products generally reduce urease activity for two to three weeks after fertilizer

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application (Gowariker and Krishnamurty 2009, Havlin et al. 2014) although under certain

conditions effectiveness can be longer (Engel et al. 2011). Urease is abundant in soils and

produced by numerous soil organisms and plants (Bremner and Douglas 1971, Bremner and Chai

1986, Antisari et al. 1996, Sanz-Cobena et al. 2008). When urease inhibitors are released to the

environment, they bind to the urease enzyme, and reduce the enzyme activity to provide time for

urea to dissolve and move into the soil. Once urea is in the soil, loss via NH3 volatilization is

reduced because the soil buffers the pH. Many compounds inhibit urease activity, but few of

these compounds are chemically stable when added to urea, effective when applied at low

concentrations and/or are non-toxic when applied in the environment (Hauck 1985, Azeem et al.

2014). The most widely used urease inhibitor is N-(n-Butyl) triphosphoric triamide (NBPT)

(Havlin et al. 2014). Urea impregnated with NBPT can reduce losses of fertilizer N via NH3

volatilization significantly compared to untreated urea (Engel et al. 2011, Raymond et al. 2016)

and may translate to improved productivity for desired species (Engel et al. 2011).

The CRN products generally have a urea granule base which is initially coated with a

compound (i.e. sulfur (S), boron, (B), copper (Cu)) and additionally sealed with a wax-like

substance to fill imperfections (Chien et al. 2009). Fertilizer N release from CRN depends on the

thickness and coating quality (Allen 1984). As the coating degrades in the environment, cracks

develop in the coating allowing for moisture to enter the product, and urea dissolution is

initiated. Urea N release to the environment is slowed by the coating compared to uncoated urea.

The substrate coating of CRN may reduce urease activity, and CRNs may also be impregnated

with urease or nitrification inhibitors to slow loss mechanisms as urea N is released to the

environment. Additionally, CRN products may include other nutrients in the coatings to address

additional nutrient deficiencies.

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An alternative CRN approach encapsulates fertilizer N (generally urea) in a semi-

permeable polymer coating. The coatings have small pores which allow water to enter the

capsules. As water enters the CRN product, urea dissolution occurs, the capsule expands, and the

resulting internal pressure of the capsule forces fertilizer N solution into the environment through

diffusion (Shaviv 2005). Because of the low mass of the polymer coatings, these products do not

significantly reduce the N percentage in the product as may occur with the aforementioned CRN

products (Chien et al. 2009). Release patterns for polymer coated CRN products are related to

the type of coating, soil moisture, temperature, relative humidity, precipitation, pH, and

microbial activity (Christianson 1988, Shoji et. al. 2001, Shaviv 2005).

1.2.7. Use of Stable Isotopes to Trace Nitrogen in Forest Ecosystems

Stable isotopes, specifically 15N, can be used to assess of nutrient deficiencies and

quantify fertilizer N uptake (Hauck 1968). Fertilizers enriched with stable isotopes (15N) can

trace the ultimate fate of applied N (Walker 1958, Hauck 1968, Powlson and Barraclough 1993)

and integrate ecosystem processes (Robinson 2001, Dawson et al. 2002, Templar et al. 2012).

Numerous 15N tracer studies were initially conducted in agroecosystems (Hauck and Bystrom

1971), but 15N tracer studies have also improved the understanding of N dynamics in natural

(Tietema et al. 1998, Currie and Nadelhoffer 1999, Dinkelmeyer et al. 2003, Nadelhoffer et al.

2004, Templar et al. 2012) and plantation (Walker 1958, Hauck 1968, Mead and Pritchett 1975,

Melin et al. 1983, Clinton and Mead 1994, Chang et al. 1996, Bubb et al. 1999, Mead et al. 2008,

Werner 2013) forests. The use of stable isotopes is often combined with other analytical methods

to improve the understanding of nutrient status and fertilizer responsiveness (Miller 1981, Birk

and Vitousek 1985, Sypert 2005).

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Fertilizers labeled with the stable isotope 15N can be traced through the ecosystem and

improve the understanding of the fate and cycling of N in forests (Peterson and Fry 1987,

Knowles and Blackburn 1993, Nadelhoffer and Fry 1994, Dawson et al. 2002, Weatherall 2005,

Templar et al. 2012). The productivity of most forests is limited by N despite relatively large

quantities of total N in the soil (Fisher and Binkley 2000). This is because only small amounts of

plant available N exist at any point in time in the ecosystem (Chapin et al. 1986, Vitousek and

Matson 1985, Fisher and Binkley 2000). The majority of N, ranging from 2 Mg ha-1 to 7 Mg ha-1, is

in the forest floor and mineral soil (Cole and Rapp 1981, Fahey et al. 1985, Likens and Bormann

1995, Fisher and Binkley 2000). Because of the large quantity of natural soil N, it is difficult to

distinguish between the N that occurs naturally in the soil and the N that originates from the

fertilizer (Nômmik 1990, Kiser and Fox 2012).

Response to fertilizer N is often assessed through changes in ecosystem metrics such as

foliar N concentration, leaf area index (LAI), C:N ratios, nutrient ratios, tree diameters, rooting

profiles, photosynthetic rates, soil respiration, soil solution measurements, and gross-net N

mineralization. Although these metrics provide valuable information on system responses to

fertilizer N, it can be difficult to discern pathways, mechanisms and partitioning of fertilizer N

within the ecosystem. Because of the difficulty in distinguishing N originating from the natural

environment or fertilizer, fertilizers can be enriched with stable isotopes (15N) to improve the

understanding of the effects of fertilization on ecosystem processes.

Elements naturally occurring in the environment have one or more stable isotopes with

slightly different molecular weights due to a difference in neutrons (Fry 2006). The ratio of

different isotopes in a compound is measured with an isotope ratio mass spectrometer (IRMS).

Current analytical techniques allow for the simultaneous measurement of the isotopic

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composition of several elements (C, N, O, H, S as examples) on an IRMS that can be coupled

with an elemental analyzer for nutrient concentration analysis on a single sample (Sulzman

2007).

During the IRMS analysis, the isotopic ratios of the compound are compared to a

standard ratio, and the difference between these ratios is expressed as the del value (δ) or per mil

(‰) (Fry 2006, Sulzman 2007) through the formula:

[1.1] δ15N = ((Rsample/Rstandard)-1) * (1000)

The Rsample is the ratio of the heavier isotope to lighter isotope of the sample, and the Rstandard is

the ratio of the heavier isotope to lighter isotope of the standard (Fry 2006, Sulzman 2007). The

standard used for N stable isotope analysis is atmospheric N2 gas which has a δ15N of 0. Samples

enriched in the heavier isotope will have positive δ value whiles samples enriched with the

lighter isotope will have negative δ values.

Two naturally occurring stable isotopes exist for N, 14N (99.63%) and 15N (0.3663%)

(Sulzman 2007). Although both isotopes behave similarly in the environment, certain kinetic and

biological reactions favor the lighter isotope (14N) in reaction products, which enriches

remaining substrate(s) in the heavier isotope (15N) in a process termed fractionation (Fry 2006).

Fractionation can create distinct isotopic signatures in ecosystem components, with δ15N values

ranging from -15 to +2, with increasingly negative values observed in vegetation and positive

values in deeper soil depths (Nadelhoffer and Fry 1994, Ben-David et al. 1998).

Nitrogen additions to ecosystems enriched with 15N improve the precision and accuracy

of tracing fertilizer N movement through the ecosystem by measuring 15N accumulation in each

ecosystem component (tree, competing vegetation, soil, etc.) (Powlson and Barrachlough 1993).

This 15N tracer technique has been used in many ecosystems to improve the understanding of N

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cycling including agriculture (Hauck 1973, Hauck and Bremner 1976), forests (Nadelhoffer and

Fry 1994, Templar et al. 2012), stream ecology (Simon et al. 2003), oceanography (Kline 1999),

and food webs (Ben-David et al.1998, Setälä et al. 2002). Yet due to the high expense of

producing 15N labeled substrates, their use is generally limited to smaller field and laboratory

experiments.

Two techniques using 15N as a tracer in forest ecosystem research are: 1) using small

amounts (few kg N ha-1) of heavily enriched (60-90 Atom Percent- AP) 15N labeled fertilizer; or

2) larger amounts (few hundred kg N ha-1) of lightly enriched (2.5-10 AP) 15N labeled fertilizer.

Forest ecosystem research has used both techniques, from highly enriched, low N concentration

additions (Koopmans et al. 1996, Seely and Lajtha 1997, Tietema et al. 1998, Perakis and Hedin

2001) to slightly enriched, higher N concentration additions (Currie and Nadelhoffer 1999,

Dinkelmeyer et al. 2003, Nadelhoffer et al. 2004). Perakis et al. (2005) applied a gradient of

enriched 15NH415NO3 to an ecosystem, ranging from 0.2 kg N ha-1 99 AP to 640 kg N ha-1 2.5 AP

in an attempt to optimize tracer techniques. Their results found that both slightly and highly

enriched substrates were detectable as tracers through the ecosystem and the decision of which

technique to use should be based more on the specific research question.

This study will use fertilizers enriched with 15N (0.5 AP, ~370‰) to trace the fate of

fertilizer N in intensively managed mid-rotation loblolly pine plantations across the southern

United States. The 15N enrichment is 20% greater than the natural abundance range for forest

ecosystem components, ranging from 0.3626 AP to 0.3718 AP (Nadelhoffer and Fry 1994). This

study balances the tracer technique suggested by Nadelhoffer and Fry (1994) by using moderate

enrichments of 0.5 AP and high N concentrations (urea, 46% N) while simulating the standard

operational fertilizer N application rate (224 kg N ha-1) in southern industrial pine plantations.

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1.3. Objectives

The objective of this study is to assess the fate of fertilizer N through the use of four

fertilizer formulations labeled with the stable isotope 15N in mid-rotation loblolly pine

plantations across the southern United States. Specific focus will be on N uptake by loblolly

pines. The four fertilizer formulations to be assessed include: 1) urea; 2) monoammonium

phosphate (MAP) coated urea + NBPT (CUF); 3) urea + NBPT (NBPT); and 4) a homogenous

polymer coated controlled release fertilizer (PCU). All fertilizer N formulations will be applied

at the industry standard of 224 kg ha-1 N combined with 28 kg ha-1 P. Two application seasons,

spring (March-April) and summer (June) will be assessed. Ammonia volatilization will be

measured following fertilization. Results from this study will provide information on the

fundamental cycling of fertilizer N in loblolly pine plantations across the southern United States

to improve fertilization techniques that will translate to improved economic efficiencies and

environmental stewardship concerning fertilization in these systems.

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11/10/2015. http://www.aapfco.org/index.html. Addoms, R.M. 1937. Nutritional studies on loblolly pine. Plant Physiol. 12(1): 199–205. Albaugh, T.J., H.L. Allen, P.M. Dougherty, L.W. Kress, and J.S. King. 1998. Leaf-area and

above- and belowground growth responses of loblolly pine to nutrient and water additions. For. Sci. 44(2):317-328.

Albaugh, T.J., H. L. Allen, P. M. Dougherty, and K. H. Johnsen. 2004. Long term growth

responses of loblolly pine to optimal nutrient and water resource availability. For. Ecol. Manage. 192:3-19.

Albaugh, T.A., H.L. Allen, and T.R. Fox. 2007. Historical patterns of forest fertilization in

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Sampson, D.A., and H.L. Allen. 1999. Regional influences of soil available water and climate, and leaf area index on simulated loblolly pine productivity. For. Ecol. Manage. 124:1-12. Sanz-Cobena, A., T.H. Misselbrook, A. Arce, J.I. Mingot, J.A. Diez, and A. Vallejo. 2008. An

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Seely, B., and K. Lajtha. 1997. Application of a 15N tracer to simulate and track the fate of

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Setälä, H., and T. Aarnio. 2002. Vertical stratification and trophic interactions among other

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Shaviv, A. 1996. Plant response and environmental aspects as affected by rate and pattern of nitrogen release from controlled release N fertilizers. In: V. Clempt et al., editors, Progress in nitrogen cycling studies. Kluwer, Netherlands. p. 285-291.

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Shrestha, R., B.D. Strahm, and E.B. Sucre. 2014. Nitrous oxide fluxes in fertilized Pinus taeda

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stands planted on Florida spodosols. Soil Sci. Soc. Am. J. 75(3):1117-1124. Volk, G.M. 1970. Gaseous loss of ammonia from prilled urea applied to slash pine. Soil Sci. Soc.

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Wells, C.G. and J.R. Jorgensen. 1979. Effect of intensive harvesting on nutrient supply and sustained productivity. In: A.L. Leaf, editor, Proceedings of the impacts of intensive harvesting on forest nutrient cycling. State University of New York, Syracuse, NY. p. 212-230.

Werner, A.T. 2013. Nitrogen release, tree uptake, and ecosystem retention in a mid-rotation

loblolly pine plantation following fertilization with 15N-enriched enhanced efficiency fertilizers. M.S. Thesis. Virginia Tech.

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fertilized with urea in the southeast US. Soil Sci. Soc. Am. J. 75:257-266.

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Figures

Figure 1.1. The nitrogen cycle in forested ecosystems. Adapted from Fisher and Binkley (2000), Dawson et al. (2002), Sylvia et al.

(2005), Groffman and Rosi-Marshall (2013).

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Figure 1.2. Soil nitrogen supply and tree nitrogen demand during a typical rotation for loblolly pine in the southern United States.

Adapted from Allen (1990) and Fox et al. (2007a).

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Figure 1.3. The fertilizer nitrogen cycle in a coniferous forested ecosystem. Adapted from Crane (1972).

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Tables

Table 1.1. Common types of nitrogen containing fertilizers for use in turf grass management, horticulture, agriculture and forestry.

Top portion of list has general applications to forestry. Information summarized from tables specific to N containing fertilizers in

Havlin et al. (2014), IPNI (2015) and PSU (2015).

Nitrogen Fertilizer Source N

(%)

P2O5

(%)

K2O

(%)

Ca

(%)

Mg

(%)

S

(%)

Cl

(%)

Application

Form

Urea (CO(NH2)2, (NH2CONH2) ) 45-46 0 0 0 0 0 0 Solid

Ammonium nitrate (NH4NO3) 33–34 0 0 0 0 0 0 Solid

Ammonium sulfate (NH4)2SO4 21 0 0 0 0 24 0 Solid

Diammonium phosphate (DAP) (NH4)2HPO4 18-21 46-54 0 0 0 0 0 Solid

Monoammonium phosphate (MAP) NH4H2PO4 10-12 48-61 0 2 2 1-3 0 Solid

Compound fertilizers 10, 12, 17, 21 10, 12, 17, 7 10, 12, 17, 14 0 0 0 0 Solid

Ammonium polyphosphate [NH4 PO3]n 10 34 0 0 0 0 0 Liquid

Potassium nitrate (KNO3) 13 0 44 0.5 0.2 0.2 1.2 Solid

UAN (urea-ammonium nitrate) (Urea + NH4NO3 + H2O)

28-32 0 0 0 0 0 0 Liquid

UAP (urea-ammonium phosphate) 21-38 13-42 0 0 0 0 0 Solid

Urea phosphate (CO(NH2)2·H3PO4) 17 43-44 0 0 0 0 0 Solid

Urea sulfate (CH6N2O5S) 30-40 0 0 0 0 6-11 0 Solid

Anhydrous ammonia (NH3) 82 0 0 0 0 0 0 Liquid-Gas

Ammonium bicarbonate (NH4HCO3) 21-23 0 0 0 0 0 0 Solid

Ammonium chloride (NH4Cl) 25-26 0 0 0 0 0 66 Solid

Ammonium thiosulfate (H8N2O3S2) 12 0 0 0 0 26 0 Liquid

Calcium ammonium nitrate (5Ca(NO3)2 ‧NH4·NO3‧10H2O)

15-27 0 0 9-19 0 0 0 Solid-Liquid

Ammonium polyphosphate (Hn+2PnO3n+1) 10-11 34-37 0 0 0 0 0 Liquid

Calcium nitrate (CAN) (Ca(NO3)2) 15 0 0 34 0 0 0 Solid

Sodium nitrate (NaNO3) 16 0 0 0 0 0 0.6 Solid

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Table 1.2. Common types of enhanced efficiency fertilizer products for use with nitrogen containing fertilizers in agriculture,

horticulture, and turf grass management. Certain products have current applications to forestry. Information summarized from tables

specific to slow and controlled release fertilizers found in Hauck (1985), Shaviv (1996), Trenkel (2010), Azeem et al. (2014), Havlin et

al. (2014), IPNI (2015) and PSU (2015). Primary resource was Table 4-23 in Havlin et al. (2014).

Enhanced Efficiency Fertilizer N Source Common Name(s) N Content (%) Inhibition Duration (weeks)

Coated

Sulfur coated urea Urea SCU

Enspan CUF

30-42 39

4-12

Polymer and/or S coated urea Urea PolyPlus Poly-S TriKote

XCU 38-42 41-43

6-16

Polymer or resin coated urea Urea

Polyon, Osmocote, Meister

Agriform Multicote Escote Prokote

ESN Nutrisphere

38-44 25-46

8-14

Insoluble Organic N

Urea formaldehyde

Ureaforms Methylene urea Methylol urea

Polymethylene urea

Nitroform FLUF

Folocron GP-4340 Nutralene Nydrolene Nitamin

Resi-Grow CoRoN

38 18 29 30 40

30 12 or 38

10-30+ 6-10

7-12

6-10 7-9

Isobutylidene diurea Isobutylidine urea IBDU 31 10-16

Triazone Triazone/urea N-Sure, Nitamin

TriSert, Formolene 28-33 6-10

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Enhanced Efficiency Fertilizer N Source Common Name(s) N Content (%) Inhibition Duration (weeks)

Crotonylidene diurea Urea/crotonaldehyde Crotodur, CDU, Triabon 34 6-12

Melamine 2,4,6 triamino- 1, 3, 5-triazine Nitrazine 50-60 6-12

DCD Dicyandiamide DCD Ensan

1.6 0

4-8

DMPP 3,4-dimethpyrazole phosphate DMPP ENTEC

0 12-26

6-8 12

Thiosulfate Ammonium or calcium

thiosulfate ATS CaTS

12 0

2-3

DCD + NBPT Dicyandiamide + N-(n-butyl)

thiophosphoric triamide Agrotain Plus HYDREXX

0 0

6-8

DCD + NBPT + urea UMAXX UFLEXX SuperU

47 46

8-12 6-8

Polymer Maleic-itaconic copolymer Nutrisphere 0 6

Polymer + urea SSN 46 6-12

Inhibitors

Nitrapyrin 2-chloro-6-trichloromethyl

pyridine N-Serve

Stay-N 2000 0 0

2-6

NBPT N-(n-butyl) thiophosphoric

triamide Agrotain SuperU

0 46

2-3

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Chapter 2. Ammonia volatilization following nitrogen fertilization with

enhanced efficiency fertilizers and urea in loblolly pine (Pinus taeda L.) plantations of the

southern United States

Abstract

Ammonia (NH3) volatilization losses following surface application of urea and three

enhanced efficiency nitrogen (N) containing fertilizers (EEFs) were compared in six

thinned mid-rotation loblolly pine stands (Pinus taeda L.) across the southern United

States. All fertilizer treatments were labeled with 15N (~370 permil, 0.5 AP) and applied

during two different seasons (spring, summer) in 2011 to open chamber microcosms.

Individual microcosms were sampled 1, 15 and 30 days after fertilization to estimate

remaining 15N. Losses of fertilizer N were determined using a mass balance calculation.

Significantly less N loss occurred following fertilization with EEFs compared to urea after

all sampling days for both seasons. Because root uptake was eliminated in the microcosms

and there was no leaching of 15N below the microcosms, the most likely loss pathway of

the 15N from the microcosms was NH3 volatilization. There were generally no differences

among the individual EEFs. Following spring application, the mean NH3 volatilization

during the 30 day experiment ranged from 4% to 26% for the EEFs compared to 26% to

40% for urea. In summer, mean NH3 volatilization for EEFs ranged from 8% to 23%

compared to 29% to 49% for urea. This research highlights the potential of EEFs to reduce

loss of fertilizer N in forest systems, potentially increasing fertilizer N use efficiency in

these pine plantations.

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2.1. Introduction

The productivity of many loblolly pine (Pinus taeda L.) plantations in the southern

United States is limited by low levels of plant available soil nutrients, especially nitrogen (N) and

phosphorous (P) (Allen 1987). These N deficiencies are common because N is required in larger

quantities compared to other nutrients for the formation of foliar tissue and photosynthetic

enzymes of cellular structure (Miller 1981, Chapin et al. 1986). Nitrogen deficiencies can

generally be ameliorated through fertilization (Fox et al. 2007a, Carlson et al. 2014). In the

South, loblolly pine plantations generally respond positively to a mid-rotation fertilization with a

mean growth increase of 3 m3 ha-1 over 8 years following the application of 224 kg ha-1 N plus

30 kg ha-1 of P (Fox et al. 2007a). Consequently, fertilization has become an important

silvicultural tool to improve forest productivity (Allen 1987, Fox et al. 2007b). Despite these

systems being N deficient, less than 50% of applied N fertilizer is usually utilized by loblolly

pines, with some studies indicating a much lower percentage (Baker et al. 1974, Mead and

Pritchett 1975, Johnson and Todd 1988, Li et al. 1991, Albaugh et al. 1998, 2004, Blazier et al.

2006).

The most common N fertilizer used in loblolly pine plantations in the South is pelletized

urea (CO(NH2)2) which is usually surface applied via aerial or ground broadcast methods (Allen

1987). Urea is used because of its high N content (46% N) and ease of transport-storage-

application, translating to the lowest overall cost per pound of applied N (Gould et al. 1986,

Allen 1987, Harre and Bridges 1988, Fox et al. 2007b). In the acidic forest soils of the South that

support loblolly pine plantations, urea undergoes a series of chemical reactions that can lead to

ammonia (NH3) volatilization losses (Hauck and Stephenson 1965).

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[2.1] (CO(NH2)2) + H+ + 2H2O urease 2NH4+ + HCO3

- (hydrolysis)

[2.2] NH4+ NH3(d) + H+ (dissociation)

[2.3] NH3(d) NH3(g) (NH3 volatilization)

The initial reaction, urea hydrolysis, is facilitated by the extracellular enzyme urease

which originates from plant and animal residues and microbial activities and is common in forest

soils (Conrad 1942, Pettit et al. 1976, Marsh et al. 2005). Urea hydrolysis produces ammonium

carbonate which dissociates into ammonium (NH4+) and bicarbonate (HCO3

-). The bicarbonate

consumes hydrogen (H+) ions near the dissolving urea granule which raises the surrounding pH.

At high pH, the ammonium ions (NH4+) dissociate and are converted to ammonia (NH3) which

can be volatilized and lost to the atmosphere.

The losses of fertilizer N by NH3 volatilization after application of urea in plantation

forests can be rapid and significant (Nômmik 1973, Kissel et al. 2004, Cabrera et al. 2010, Zerpa

and Fox 2011, Elliot and Fox 2014). Losses range from less than 10% (Boomsma and Pritchett

1979, Craig and Wollum 1982), 10% to 40% (Nômmik 1973, Zerpa and Fox 2011) to more than

50% (Kissel et al. 2004, Kissel et al. 2009, Elliot and Fox 2014). The losses due to NH3

volatilization in pine plantation systems are similar to the NH3 volatilization losses observed in

agriculture, which can range from 25% to 47% when urea is applied to the soil surface (Scharf

and Alley 1988).

The factors that influence NH3 volatilization after urea fertilization in agricultural

systems are well studied (Volk 1959, Black et al. 1987, Kissel et al. 1988) whereas less research

has focused on forested systems (Volk 1970, Nômmik 1973, Kissel et al. 2004, Elliot and Fox

2014). Ammonia volatilization is affected by soil pH (Ernst and Massey 1960, Cabrera et al.

1991, Kissel et al. 2009), soil moisture (Clay et al. 1990, Kissel et al. 2004), mineral soil

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substrate (Cabrera et al. 2005, Kissel et al. 2009, Zerpa and Fox 2011), relative humidity

(Cabrera et al. 2005), soil temperature (Ernst and Massey 1960, Clay et al. 1990, Moyo et al.

1989), surficial wind speed (Watkins et al. 1972, Kissel et al. 2004), precipitation (Craig and

Wollum 1982, Kissel et al. 2004) and air temperature (Gould et al. 1973, Craig and Wollum

1982, Koelliker and Kissel, 1988). The organic (O) horizon (forest floor) in forest soils can also

have a significant effect on NH3 volatilization (Cabrera et al. 2005, Kissel et al. 2009, Zerpa and

Fox 2011). Soils with a high H+ buffering capacity (high organic matter, clay, silt) generally

have lower NH3 volatilization losses of fertilizer N compared to those soils with a lower

buffering capacity (sand) (Fenn and Kissel 1976, Ferguson et al. 1984).

Urea applied under cooler, wetter conditions generally has low NH3 volatilization losses

(Ferguson and Kissel 1986, Moyo et al. 1989, Cabrera et al. 2010) due to rapid urea dissolution

and movement into the soil (Black et al. 1987, Paramasivan and Alva 1997). Higher

temperatures and relative humidity stimulate urease activity and increase NH3 volatilization

(Craig and Wollum 1982, Ferguson and Kissel 1986, Moyo et al. 1989). Elevated wind near the

soil boundary layer (Kissel et al. 2004) can also exacerbate losses. Higher rates of NH3

volatilization also occur with higher pH values (Koelliker and Kissel 1988). The ammonium ions

may enter the soil if a precipitation event occurs soon after fertilization which decreases NH3

volatilization losses (Kissel et al. 2004). The amount of urea N lost from the system due to NH3

volatilization is difficult to accurately predict because the loss of fertilizer N is driven by the

interaction among many of these factors. Large losses of fertilizer N through NH3 volatilization

have occurred even under low temperatures (Carmona and Byrnes 1990, Engel et al. 2011).

Conversely, Kissel et al. (2004) observed low NH3 volatilization during August when urea was

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applied on a day with significant precipitation but high NH3 volatilization (45% to 58%) under

simulated, minor precipitation events.

Enhanced efficiency fertilizers (EEFs) have been developed to minimize losses through

NH3 volatilization (Hauck 1985, Goertz 1993, Azeem et al. 2014). Enhanced efficiency

fertilizers that reduce NH3 volatilization can be divided into two broad categories (Azeem et al.

2014). In the first, a chemical additive, such as N-(n-butyl) thiophosphoric triamide (NBPT)

impregnates the urea granule which reduces urease activity near the urea granule (Bremner and

Douglas 1971, Bremner and Chai 1986, Antisari et al. 1996, Sanz-Cobena et al. 2008). Reducing

urease activity allows the urea granule to dissociate and slowly move into the soil, thus reducing

NH3 volatilization losses (Bremner and Douglas 1971). A second EEF method is to coat urea

granules with a physical barrier, as with a sulfur (S) or a polymer coating. This approach slows

dissolution of the urea granule so that it is released to the environment in a more constant,

gradual rate. This may reduce NH3 volatilization losses and create release rates more

synchronous with plant demand during the year (Shaviv 1996, Blazier et al. 2006).

The primary objective of this research was to determine the effectiveness of three

enhanced efficiency fertilizers compared to urea in reducing fertilizer N volatilization losses in

mid-rotation loblolly pine plantation systems in the South. We compared NH3 volatilization

losses following fertilization in two different seasons (spring, summer). Two statistical

hypotheses were tested in this experiment:

HO1: There are no differences in NH3 volatilization between urea and enhanced efficiency

fertilizers.

HO2: There are no differences in NH3 volatilization for treatments between seasonal applications

(spring, summer).

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2.2. Materials and Methods

2.2.1. Experimental Design

The experiment used a split plot complete block design to test differences in NH3

volatilization losses. Four fertilizer treatments (main plots) were applied at two different seasons,

spring versus summer (split plot) following fertilization in mid-rotation loblolly pine plantations.

The fertilizer treatments were the main plots with a single replication of each treatment

combination (fertilizer source and season of application) at each individual site. The six sites

served as blocks and provided replication. The split plot was the application date (spring,

summer).

2.2.2. Fertilizer Treatments

The four fertilizer treatments were: 1) urea; 2) urea impregnated with N-(n-Butyl)

thiophosphoric triamide (NBPT); 3) urea impregnated with N-(n-Butyl) thiophosphoric triamide

and coated with monoammonium phosphate and a proprietary binder (CUF); and 4) polymer

coated urea (PCU). Urea (46-0-0) was the conventional N fertilizer used because of its common

use in southern forests. The enhanced efficiency fertilizers (EEFs) tested in this study were

developed to reduce NH3 volatilization and release fertilizer N slowly to the environment. In the

NBPT treatment, urea (46-0-0) granules were impregnated with N-(n-butyl) thiophosphoric

triamide at a rate of 26.7% by weight to inhibit urease activity. In the CUF treatment (39-9-0),

urea granules were impregnated with NBPT and coated with an aqueous binder containing a

boron and copper sulfate solution to slow the release of N to the environment. A coating of

monoammonium phosphate was then added to provide P. The PCU (44-0-0) treatment had a

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polymer coating covering urea granules with pore openings designed to slowly release N (~80%)

over a 120 day period. The application rate for all treatments was equivalent to 224 kg N ha-1.

Because the CUF treatment contained P in the monoammonium phosphate coating, P was added

to the other fertilizer treatment at the equivalent rate of 28 kg P ha-1 using triple superphosphate

(TSP). The urea for all treatments was enriched with the stable isotope 15N (~370‰, 0.5 AP)

(Nadelhoffer and Frye 1994). All individual fertilizer treatments for both studies were applied by

hand to individual microcosms.

2.2.3. Site Descriptions

Six sites were selected adjacent to plots in an existing network of forest thinning and

fertilization studies in mid-rotation loblolly pine plantations across the South (Figure 2.1).

Selected site characteristics are detailed in Tables 2.1 and 2.2 with selected soil chemical and

physical data in Table 2.3. Daily mean weather data for study sites was obtained from the Oak

Ridge National Laboratory (ORNL) DAYMET website (http://daymet.ornl.gov/) and nearest

weather stations from the Weather Underground website (http://www.wunderground.com/).

2.2.4. Experimental Method

Ammonia volatilization losses were determined using an open chamber microcosm

methodology detailed by Marshall and Debell (1980) and May and Carlyle (2005). Each

microcosm was constructed from a white Schedule 40 polyvinyl chloride pipe (PVC) with an

inner diameter of 15.24 cm (Figure 2.2). Each microcosm was inserted vertically into the soil

through the entire organic (O) horizon (Oi + Oe + Oa) into the mineral soil to a depth of 30 cm

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which severed roots. A 2.5 cm portion of the microcosm remained above the O horizon. The O

and mineral soil horizons inside the microcosm were minimally disturbed during insertion.

Immediately prior to each treatment application to individual microcosms, two fertilizer

granules were randomly sampled and placed in labeled scintillation vials for 15N analysis that

was used in the overall mass balance calculation for each individual treatment determination of

total 15N recovery. The four fertilizer treatments were applied to the surface of the O horizon in

each microcosm. The fertilizer N treatments were applied to randomly selected microcosms on

two separate dates. All fertilizer applications in a specific season were made on a single day at

individual sites. The spring application dates ranged from March 26 to April 8, 2011, and the

summer application dates ranged from June 18 to June 30, 2011. At each site one microcosm for

each fertilizer treatment was randomly selected and removed without disturbing the soil inside

the microcosm on 1, 15 or 30 days after fertilizer application. An additional mineral soil sample

was taken immediately below the microcosm at each sampling date to determine if fertilizer 15N

had leached below the microcosm. After removal, a plastic cover was secured to the top of the

microcosm and plastic wrap to the bottom, and the intact microcosms were placed on ice in a

cooler and transported to the laboratory for processing.

2.2.5. Laboratory Processing

All microcosms were stored in a walk-in freezer with a constant temperature of -20.0°C

until processed. Upon removal from the freezer, microcosms were thawed and the soil was

divided into four depth increments: O horizon (Oi + Oe + Oa) and the mineral soil increments of

0-10 cm, 10-20 cm, and 20-30 cm. Each depth increment from each microcosm was wet sieved,

placed in a labeled double paper bag and dried in a forced air walk-in oven with a constant

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temperature of 60°C for 1 week. After drying, O horizon samples were sieved through a 6 mm

sieve whereas mineral soil samples were sieved through a 2 mm sieve. All sieved O horizon and

mineral sieved solid soil samples were ground to a fine powder in a ball mill (Retsch® Mixer

Mill MM 200, Haan, Germany) for 2 minutes at 25 rps to ensure proper homogenization.

Between 40 mg to 45 mg of the ball milled samples were weighed in a tin capsule on a Mettler-

Toledo© MX5 microbalance (Mettler-Toledo, Inc., Columbus, OH, USA). The 15N/14N isotopic

ratio and total N of individual samples were analyzed on a coupled elemental analysis-isotope

ratio mass spectrometer (IsoPrime 100 EA-IRMS, Isoprime© Ltd., Manchester, UK). All

instruments prior to isotopic-elemental analysis were cleaned with an ethanol solution and

allowed to dry after each sample was processed to reduce potential contamination. The quantity

of fertilizer N recovered in each depth increment was determined from the 15N/14N isotope ratio

and total N using the method discussed by Nadelhoffer and Fry (1994). Fertilizer recovered in

each depth increment in the microcosm was summed to determine the total recovery in the

microcosm. The fertilizer N not recovered was attributed to NH3 volatilization loss, and

expressed as a percentage of the applied fertilizer N.

2.2.6. Weather Data

Daily weather data, including mean daily temperature and total daily precipitation during

the first 15 days of the experiment was downloaded from the nearest weather station to each site

through the Oak Ridge National Laboratory DAYMET website (http://daymet.ornl.gov/) and the

Weather Underground (http://wunderground.com).

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2.2.7. Calculations

The individual total fertilizer N recovery for each depth increment in each microcosm

was determined as detailed below, and all depth increments for each individual microcosm was

summed to obtain a total fertilizer recovery for each individual microcosm. The individual depth

increment in each microcosm was calculated using individual soil depth mass, N concentration,

and 15N (‰) adapted from Powlson and Barraclough (1993), Nadelhoffer and Fry (1994) and

Nadelhoffer et al. (1995). Equation [3.1] calculated the ratio of fertilizer 15N to natural

abundance 15N:

Equation [3.1] was used to calculate the individual soil depth increment component N percent

that originated from the fertilizer N:

[3.1] %N from fertilizer in individual soil depth increment =

((δ15Nfinal - δ15Ninitial) / (δ15Nlabeled - δ15Ninitial)) * (100)

Equations [3.2] to [3.5] determined the percent recovery of fertilizer N in individual soil depth

increments:

[3.2] % 15N from labeled treatment in individual soil depth increment =

((δ15Ncomponent - δ15Npretreatment) / (δ15Nfertlizer - δ15Npretreatment)) * (100)

[3.3] Total N (g) = (Individual soil depth increment component dry weight * %N) / (100)

[3.4] N originated from fertilizer (g) in individual soil depth increment component =

( (Total N * %15N originated from fertilizer) / (100) )

[3.5] % N originated from fertilizer in individual soil depth increment =

[(N from fertilizer (g) in individual soil depth increment)/(15N fertilizer (g) ) ] * (100)

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2.2.8. Statistical Analysis

Ammonia volatilization loss (%), expressed as a percentage of fertilizer N not recovered

from an individual microcosm, was analyzed using a repeated measure analysis of variance

(ANOVA) using PROC MIXED in SAS® 9.4 (SAS Institute, Cary, NC). Several alternative

spatial covariate structures were tested, and the optimal structure was the unstructured covariate

structure based on Akaike’s Information Criteria (AIC). Percent data were arcsin transformed

prior to analysis (Gomez and Gomez 1984). Ammonia volatilization loss (%) was the response

variable for the model, day (1, 15, 30) was the repeated measure, fertilizer treatment (CUF,

NBPT, PCU, Urea) and season (spring, summer) were fixed effects, and site (VA, ALN, AR,

ALS, SC, NC) was a random effect. (Littell et al. 2006). Levels of significance were set at α =

0.05 and the P > |t| values for the treatment means for each day and season were tested using the

SLICE function in PROC MIXED. The post-hoc analysis used was Tukey’s HSD.

Pearson’s correlations were conducted between treatments and weather variables to

assess relationships with NH3 volatilization loss. Maximum daily relative humidity and mean

daily relative humidity were calculated from dewpoint and temperature data (Iduchov and

Eskridge 1996), mean daily critical relative humidity (CRH) for urea dissolution was calculated

using the procedure in Elliot and Fox (2014). The number of days when mean daily relative

humidity was greater than the CRH for each sampling interval (1, 15, 30 days after fertilization)

were determined. Pearson’s correlation coefficients were calculated for mean NH3 volatilization

from urea and the selected weather data 1 day and 15 consecutive days after fertilization for

spring, summer and spring + summer application seasons combined to assess relationships

between NH3 volatilization from urea and weather variables. Strong correlations were defined as

0.5 < |r|, moderate correlations as 0.3 < |r| < 0.5, and small correlations as 0.1 < |r| < 0.3.

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2.3. Results

2.3.1. Differences in NH3 Volatilization Among Fertilizer Treatments – Spring

Mean NH3 volatilization losses following urea application in the spring were greater than

losses from the EEFs after 1, 15 and 30 days after fertilizer application (Table 2.6; Figure 2.4).

There were no significant differences among the individual EEFs 1 and 15 days after

fertilization, but mean NH3 volatilization losses following CUF application were less than mean

NH3 volatilization losses from NBPT and PCU after 30 days. Mean NH3 volatilization losses,

expressed as a percent of the fertilizer N added, after 1 day were 3.9% for CUF, 9.6% for NBPT,

8.2% for PCU and 25.9% for urea. Fifteen days after fertilization, the mean NH3 volatilization

was 13.7% for CUF, 20.0% for NBPT, 15.3% for PCU, and 35.2% for urea. Thirty days after

fertilization, the mean NH3 volatilization was 15.2% for CUF, 23.5% for NBPT, 21.5% for PCU

and 40.0% for urea.

2.3.2 Differences in NH3 Volatilization Among Fertilizer Treatments – Summer

Mean NH3 volatilization losses following urea fertilization in the summer were

significantly greater after 1, 15 and 30 days then mean NH3 volatilization losses from the EEFs

(CUF, NBPT, PCU) except after 1 day, where there was no difference between CUF and urea

(Table 2.6; Figure 2.4). There were no significant differences in mean NH3 volatilization among

the individual EEFs after 1 and 15 days, but after 30 days losses from PCU were less than CUF.

Mean NH3 volatilization losses, expressed as a percent of fertilizer N added, were 18.8% for

CUF, 6.7% for NBPT, 11.0% for PCU and 28.5% for urea after 1 day. After 15 days, the mean

NH3 volatilization was 16.6% for CUF, 14.2% for NBPT, 11.4% for PCU, and 45.4% for urea.

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After 30 days, the mean NH3 volatilization was 23.3% for CUF, 21.4% for NBPT, 11.8% for

PCU and 48.7% for urea.

2.3.3. NH3 Volatilization Among Fertilizer Treatments – Spring + Summer

When the data was combined for the spring and summer application (spring + summer),

mean NH3 volatilization losses were significantly greater following fertilization with urea than

the EEFs after 1, 15 and 30 days (Table 2.6; Figure 2.4). There were no significant differences

among the individual EEFs 1 and 15 days after fertilization, but mean NH3 volatilization losses

were greater following fertilization with NBPT than PCU after 30 days. Mean NH3 volatilization

losses, expressed as a percent of fertilizer N added, were 11.3% for CUF, 8.7% for NBPT, 9.6%

for PCU and 27.3% for urea after 1 day. The mean NH3 volatilization was 15.2% for CUF,

17.1% for NBPT, 13.9% for PCU, and 40.3% for urea after 15 days. After 30 days, the mean

NH3 volatilization was 19.2% for CUF, 22.4% for NBPT, 16.7% for PCU and 44.4% for urea.

2.3.4 Correlation Between NH3 Volatilization From Urea and Weather Data

Correlations between weather data and cumulative mean NH3 volatilization loss,

expressed as a percentage of N applied, following fertilization with urea for spring, summer and

spring + summer data were generally poor (Table 2.7, Appendix Figure A.2.1.). After the spring

fertilization, a strong negative correlation occurred with total daily precipitation (-0.512),

moderate positive correlations with maximum daily temperature (0.312) and mean daily

temperature (0.303), and a moderate negative correlation when the critical relative humidity

exceeded the relative humidity (-0.303). After the summer fertilization, strong positive

correlation occurred with mean daily relative humidity, and moderate positive correlations with

maximum daily relative humidity (0.375) and minimum daily relative humidity (0.375). When

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the spring and summer data was combined, only a moderate negative correlation occurred with

total daily precipitation (-0.311).

Ammonia volatilization losses from urea 15 days following the spring application were

strongly correlated with maximum daily temperature (0.579), mean daily temperature (0.808)

and cumulative days when daily mean relative humidity exceeded critical relative humidity

(0.697). Moderate negative correlations occurred with maximum daily relative humidity

(-0.406), and a positive correlation with mean daily relative humidity (0.470). Following summer

application, no strong correlations were observed. Moderate positive correlations existed with

maximum daily relative humidity (0.403) and mean daily relative humidity (0.322). When data

for mean NH3 volatilization losses after 15 days following spring and summer applications were

combined, a strong positive correlation existed with the cumulative days when mean daily

relative humidity was greater than the critical relative humidity (0.573), and moderate positive

correlations existed with maximum daily temperature (0.406) and cumulative precipitation

(0.386).

2.4. Discussion

Our study focused on assessing NH3 volatilization following N fertilization with urea and

enhanced efficiency N containing fertilizers in loblolly pine plantations in the southern United

States. We correlated weather variables associated with NH3 volatilization loss of the fertilizer N

(Ernst and Massey 1960, Craig and Wollum 1982, Cabrera et al. 2005). By installing a study

across the entire region where loblolly pine plantations are operationally grown, we hoped to

improve the understanding of the magnitude of fertilizer N loss across the region rather than

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focus on a detailed study in a single location or smaller area as done by others (Volk 1970,

Nômmik 1973, Kissel et al. 2004, Elliot and Fox 2014). This study tested two primary

hypotheses: 1) whether differences existed in NH3 volatilization between urea and enhanced

efficiency fertilizers; and 2) whether differences existed in NH3 volatilization among treatments

between application seasons.

Because tree roots were severed when the microcosms were inserted into the soil, N

uptake by the trees was eliminated. Therefore, we assume that the primary loss mechanism for

the applied N from the open chamber microcosm was NH3 volatilization to the atmosphere

(Figure 2.2). However, the two additional potential loss pathways that could occur from the

microcosms are leaching and denitrification. We tested for potential leaching loss of fertilizer N

by sampling soil directly below each microcosm on each date and analyzing the sample for 15N.

If this loss pathway was important, samples immediately below the bottom of the microcosm

(>30 cm soil increment) should have had elevated 15N signatures. The 15N signature (‰) for the

>30 cm increment was similar to the natural abundance background 15N values at each site

(Table 2.4). We were unable to detect any of the fertilizer 15N in the >30 cm depth increment

after any sampling date (1, 15, 30) following the spring or summer fertilization with any

fertilizer (CUF, NBPT, PCU, urea). This result indicates that leaching was an undetectable loss

pathway in this study (Figure 2.3).

We did not directly measure denitrification losses in this study. Recent work examining

denitrification on similar sites used in this study found that denitrification losses were relative

low during spring and summer months (Shrestha et al. 2014). Denitrification occurs in anaerobic

environments when nitrate functions as the terminal electron acceptor during the oxidation of

soil organic matter. Microcosms in this study were all located in aerobic soil. Even at the sites

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with poorly drained soils, microcosms were located on top of the beds where soils were aerobic

(Kelting et al. 1998, Shrestha et al. 2014). We compared N losses on well versus poorly drained

soils in our study and found no differences in percent recovery of fertilizer N due to soil drainage

class. Since losses were similar on both types of soil, denitrification was a minor loss mechanism

in this study. Although a small amount of fertilizer N loss may have occurred through both

denitrification and leaching, we concluded that NH3 volatilization was the primary loss pathway

for N in our microcosm systems.

Our first hypothesis that no differences in NH3 volatilization existed between urea and

enhanced efficiency fertilizers was rejected. Ammonia volatilization losses were significantly

lower for enhanced efficiency fertilizers (CUF, NBPT, PCU) compared to urea following both a

spring and summer application. Significantly less NH3 volatilization occurred with all enhanced

efficiency fertilizers, and when differences were present among individual enhanced efficiency

fertilizers they were small. Yet with urea, NH3 volatilization losses were large following

fertilization in both spring and summer, ranging from 25% to 30% of fertilizer N lost after a

single day. Losses following fertilization with EEFs were much less compared to urea. Ammonia

volatilization losses continued to increase through time for all fertilizers. The incremental

additional loss through 15 days and 30 days after fertilization were greater for urea than the

EEFs. The result for these fertilizer treatments agree with those of other studies which have

generally found the largest losses for NH3 volatilization for urea occurring between 1 day and 7

days after application (Craig and Wollum 1982, Clay et al. 1990, Kissel et al. 2009, Elliot and

Fox 2014).

The major loss pathway of the fertilizer N for this study was assumed to be through NH3

volatilization. Urea hydrolysis, facilitated by urease, produces ammonium carbonate which

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dissociates into ammonium (NH4+) and bicarbonate (HCO3

-). The bicarbonate consumes

hydrogen (H+) ions near the dissolving urea granule which raises the surrounding pH (Hauck and

Stevenson 1965). In this situation the ammonium ions (NH4+) can be converted to ammonia

(NH3) which can be lost to the atmosphere. Higher rates of precipitation immediately following

fertilization with urea, in excess of 1.27 cm, can dissolve the urea and move it into the soil,

reducing NH3 volatilization (Kissel et al. 2004). Sites for this study had minor (less than 12 mm)

or no precipitation events within the first few days following fertilization. Minor precipitation

events coinciding with warm temperatures following urea fertilization can exacerbate losses

through the NH3 volatilization pathway (Kissel et al. 2004) which occurred in our study and

likely caused large losses for urea. Conversely, studies have shown large precipitation events

after fertilization with urea can move fertilizer N into the soil profile rapidly (Kissel et al. 2004).

Once the fertilizer N moves into the soil profile, it becomes more difficult for NH4+ conversion

to NH3 due to the soil buffering capacity (Ferguson et al. 1984) and mass flow of NH3(g) to the

atmosphere. Two enhanced efficiency fertilizers we used in this study, CUF and NBPT, were

developed to reduce urease activity for a short period (approximately 2 weeks), allowing time for

precipitation to move the urea into the soil and thereby reducing losses from NH3 volatilization.

For the PCU treatment, water from the minor precipitation events may have been moving into

the PCU granules, or N release may have just been initiated from the granule with incomplete

dissolution of the urea granule occurring at the time of the analysis.

We rejected our second hypothesis that there were no differences in NH3 volatilization

between application seasons (spring versus summer). There was a significant interaction effect

between treatment and season indicating larger NH3 volatilization losses following the summer

application (Table 2.5). Losses from urea were greater in the summer compared to spring. Sites

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generally experienced higher temperatures, more days when relative humidity exceeded critical

relative humidity, and few large precipitation events. As previously discussed, these weather

conditions increase the chances for NH3 volatilization from urea because the rate of urease

activity increases (Koelliker and Kissel 1988, Moyo et al. 1989, Kissel et al. 2004). When there

is insufficient precipitation to move dissolved urea into the soil, NH3 can be lost to the

atmosphere, or losses may be exacerbated by minor precipitation events (Kissel et al. 2004).

Differences among the EEFs were small and somewhat variable. For day 1 in the

summer, losses from the CUF treatment were greater compared to spring, but similar after 15

days and 30 days. For the NBPT and PCU treatments, losses were less each day for the summer

compared to the spring. When reviewing site specific data, there were more frequent, smaller

precipitation events during the summer compared to the spring at most sites. Although this can

increase NH3 volatilization from urea (Kissel et al. 2004), this may assist in gradually moving

the EEFs into the soil due to their release mechanisms. The CUF and NBPT treatments generally

have a two week effectiveness period for minimizing urease activity (Shaviv 1996). The PCU

treatment is formulated to allow moisture into the polymer coating which permits gradual urea

dissolution. As moisture enters the PCU granule, it expands and dissolved urea-N is forced

through the polymer coating, gradually releasing fertilizer N to the environment. For the summer

application period, more frequent, smaller precipitation events may have increased in the release

and movement of N into the soil for PCU.

An additional question for this research was the effect of weather on NH3 volatilization

for urea. Similar to others (Cabrera et al. 2005, Cabrera et al. 2010, Elliot and Fox 2014) we

found a strong correlation between NH3 volatilization from urea and the number of days which

relative humidity exceeded the calculated critical relative humidity during the first days

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following fertilization (Elliot and Fox 2014). When relative humidity exceeds the threshold of

critical relative humidity, urea hydrolysis occurs and can lead to NH3 volatilization (Cabrera et

al. 2005).

The amount of urea N lost from the system due to NH3 volatilization is difficult to

accurately predict because the loss of fertilizer N is affected by many factors. High surficial wind

speed (Watkins et al. 1972, Kissel et al. 2004), low precipitation (Craig and Wollum 1982, Kissel

et al. 2004) and high air temperature (Gould et al. 1973, Craig and Wollum 1982, Koelliker and

Kissel 1988) increase NH3 volatilization. Urea applied under wetter, cooler periods tends to

reduce NH3 volatilization losses (Ferguson and Kissel 1986, Moyo et al. 1989, Cabrera et al.

2010) due to rapid urea dissolution (Black et al. 1987, Paramasivan and Alva 1997) which moves

N into the soil compared to urea remaining on the soil surface. This is a primary reason why urea

is applied during winter months in loblolly pine plantations in the South. However, large losses

of fertilizer N through NH3 volatilization have occurred even under low temperatures (Carmona

and Byrnes 1990, Engel et al. 2011). High temperatures and high relative humidity can stimulate

urease activity increasing NH3 volatilization (Craig and Wollum 1982, Ferguson and Kissel

1986, Moyo et al. 1989). Conversely, Kissel et al. (2004) observed low NH3 volatilization during

August when urea was applied on the same day as a significant precipitation event, but high

volatilization (45% to 58%) under simulated, minor precipitation events. These weather variables

were also positively correlated with large NH3 volatilization losses in our study.

Interactions of weather variables with edaphic factors may also increase NH3

volatilization and can include increases with soil pH (Ernst and Massey 1960, Koelliker and

Kissel 1988, Cabrera et al. 1991, Kissel et al. 2009), soil moisture (Clay et al. 1990, Kissel et al.

2004), type of mineral soil substrate (Cabrera et al. 2005, Kissel et al. 2009, Zerpa and Fox

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2011), low soil buffering capacity as in sand or soils with low organic matter (Fenn and Kissel

1976, Ferguson et al. 1984), and soil temperature (Ernst and Massey 1960, Clay et al. 1990,

Moyo et al. 1989, He et al 1999). Although similarities exist between agriculture and forest

systems, important differences occur which may also affect NH3 volatilization losses between

the two systems. One important difference is the presence of an organic (O) horizon (forest floor)

in forest soils which can have a significant effect on NH3 volatilization (Cabrera et al. 2005,

Kissel et al. 2009, Zerpa and Fox 2011). Although we did not measure many of these factors,

future research should continue to investigate their interactions to refine our understanding of

NH3 volatilization losses of urea from forest ecosystems.

Based on our findings, the primary fertilizer N loss pathway in this study was from NH3

volatilization. In acidic, aerobic forest soils, denitrification is considered to be a minor loss

pathway for N (Bowden et al. 1990) except in anaerobic soils. Although anaerobic conditions

may have existed in the interbed areas of three of our sites, all the microcosms were installed in

the bedded areas which were likely aerobic (Kelting et al. 1998). Interbed areas of a site can have

extended periods of anaerobic conditions, and the pathway of fertilizer N loss may increasingly

shift towards denitrification (Shrestha et al. 2014). Because fertilizers are generally applied

through surface broadcast methods, and in essence 50% of the site may be comprised of

interbeds, understanding loss mechanisms of fertilizer N in these areas is important.

Additionally, the organic horizon is generally thicker in the interbed compared to the bed, and

this potentially influences the amount of fertilizer N loss that can occur (Cabrera et al. 2005).

Our analysis of fertilizer N loss based on the grouping of well drained (aerobic) compared to

poorly drained soils did not find any differences, indicating soil drainage classification for the

bedded areas of the sites were not a controlling variable for fertilizer N loss. If losses were

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different between these two soil groupings, losses of fertilizer N by denitrification in the poorly

drained sites would need to be considered.

The remaining potential loss pathway from the microcosms was leaching. Measurements

immediately adjacent and outside the bottom opening of the microcosm found no detectable

increase in the 15N values in the soil depth of >30 cm compared to natural abundance values of a

control. From these findings, we conclude the majority of fertilizer N loss was by the NH3

volatilization pathway. If other losses did occur from the system, they were below the detectable

limits of our methods. We did not measure NH3 volatilization from unfertilized control plots for

a comparison with treatment plots due to traditionally low observed values in forest ecosystems

by others (Marshall and DeBell 1980, Kissel and Cabrera 1988, Elliot and Fox 2014).

A primary benefit of this study is the results are drawn from an extensive geographic

area, encompassing the entire region where loblolly pine plantations are operationally fertilized.

Because this study does encompass such an extensive region, and hence large range of climatic

and edaphic variables, a synthesis of significant factors helps to support the results specific to

NH3 volatilization after N fertilization of other studies in forest ecosystems that are an

assessment of a single or smaller regional sites. Additionally, the open chamber system design of

the microcosm does not have the potential limitations of closed chamber systems that can

manipulate pressure, temperature or wind speed, although the results of this study still should be

viewed as an index and not absolute values. This study provides continued evidence that

although weather conditions can be important in determining NH3 volatilization losses from

urea, they do not translate to the same losses for enhanced efficiency fertilizers. These results

provide evidence to forest managers that enhanced efficiency fertilizers can significantly reduce

NH3 volatilization losses of fertilizer N across the entire range of southern loblolly plantations,

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regardless of the season of application. The use of enhanced efficiency N containing fertilizers

may translate to improved forest productivity and fertilizer N use efficiency in southern loblolly

pine plantations in the future.

2.5. Conclusions

The primary results for this research were that significantly lower NH3 volatilization

losses occurred between all forms of N containing enhanced efficiency fertilizers (CUF, NBPT,

PCU) over extensive ecophysiographic regions of the southern United States in mid-rotation

loblolly pine stands compared to the conventional current standard of forest fertilization, urea.

These results indicate that enhanced efficiency N containing fertilizers can reduce NH3

volatilization losses over a significant range of weather and edaphic factors compared to urea.

These findings may translate to improved fertilizer N use efficiency in southern loblolly pine

plantation systems by retaining more fertilizer N in the system, with the possibility of reducing

application rates of N in the future. These findings are important to the continued direction of

improving productivity in southern loblolly pine plantations. Using enhanced efficiency

fertilizers may also provide the ability for forest managers to apply fertilizer N in a more

synchronous manner to plant demand under conditions where large NH3 volatilization losses

occur with urea, continuing to improve fertilizer N use efficiency in these systems. Viewed

collectively, these results indicate that NH3 volatilization can be decreased significantly using

any of the enhanced efficiency fertilizers in this study (CUF, NBPT, PCU) when compared to

urea under a diverse range of climatic and site variables for pine plantations in the southern

United States. These results provide forest managers a level of certainty that if NH3

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volatilization is a concern at the site, enhanced efficiency N containing fertilizers may assist in

retaining more fertilizer N in the system, translating to a potentially higher fertilizer N use

efficiency over the rotation of the stand.

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Acknowledgments

This research was primarily supported by the Forest Productivity Cooperative (FPC), the

National Science Foundation Center for Advanced Forest Systems (CAFS) and the United States

Department of Agriculture National Institute of Food and Agriculture McIntire-Stennis Capacity

Grant. We thank David Mitchem for assistance in the laboratory, Andy Laviner and Eric

Carbaugh for field assistance, and the numerous work study and seasonal student employees for

both field and laboratory assistance.

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Fenn, L.B., and D.E. Kissel. 1976. The influence of cation exchange capacity and depth of incorporation on ammonia volatilization from ammonium compounds applied to calcareous soils. Soil Sci. Soc. Am. J. 40(3):394-398.

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applied urea. Soil Sci. Soc. Am. J. 50(2):485–490. Fox, T.R., H. L. Allen, T. J. Albaugh, R. Rubilar, and C.A. Carlson. 2007a. Tree nutrition and

forest fertilization of pine plantations in the southern United States. South. J. Appl. For. 31(1):5-11.

Fox, T.R., E.J. Jokela, and H.L. Allen. 2007b. The development of pine plantation silviculture in

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oxidizing bacteria as related to the fate of 14C and 15N labeled urea added to soil. Biol Fertil Soils. 42:137-145. DOI 10.1007/s00374-005-0004-2.

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losses of nitrogen following urea fertilization of forest soils. Can. J. Soil Sci. 60:549-563. May, B.M. and C.J. Carlyle. 2005. Nitrogen volatilization from urea fertilizer in mid-rotation

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of N and P and N movement in the soil. Plant Soil 43:451-465. Miller, H.G. 1981. Forest fertilization: some guiding concepts. Forestry. 54(2):157-157. Moyo, C.C., D. Kissel, and M.L. Cabrera. 1989. Temperature effects on soil urease activity. Soil

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fate of 15 N-labelled nitrate additions to a northern hardwood forest in eastern Maine, USA. Oecologia 103:292-301.

Nômmik, H. 1973. The effect of pellet size on the ammonia loss from urea applied to forest soil.

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fertilizers. Comm. Soil Sci. Plant Anal. 28(17-18):1663-1674.

Pettit, N.M., A.R.J. Smith, R.B. Freedman, and R.G. Burns. 1976. Soil urease: Activity, stability and kinetic properties. Soil Biol. Biochem. 8(6):479–484.

Powlson, D.A., and D. Barraclough. 1993. Mineralisation and assimilation in soil-plant systems. Nitrogen isotope techniques. Eds. R.Knowles and H.Blackburn. pp. 209–221. Academic Press, San Diego, California, USA.

Sanz-Cobena, A., T.H. Misselbrook, A. Arce, J.I. Mingot, J.A. Diez, and A. Vallejo. 2008. An

inhibitor of urease activity effectively reduces ammonia emissions from soil treated with urea under Mediterranean conditions. Agr. Eco. Environ. 126:243–249.

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nitrogen release from controlled release N fertilizers. In: Progress in nitrogen cycling studies. Van Clempt (Ed.). Kluwer. Netherlands. p. 285-291.

Shrestha, R., B.D. Strahm, and E.B. Sucre. 2014. Nitrous oxide fluxes in fertilized Pinus taeda plantations across a gradient of soil drainage classes. J. Envir. Qual. 43:1823–1832.

Volk, G. M. 1959. Volatile loss of ammonia following surface application of urea to turf or bare

soil. Agr. Journal. 99(12):746-749. Volk, G.M. 1970. Gaseous loss of ammonia from prilled urea applied to slash pine. Soil Sci. Soc.

Am. J. 34:513-516. Watkins, S. H, R. F Strand, D.S DeBell, and J. Esch. 1972. Factors influencing ammonia losses

from urea applied to Northwestern Forest soils. Soil Sci. Soc. Am. Proc. 36(2):354-357. Zerpa, J.L. and T.R. Fox. 2011. Controls on volatile NH3 losses from loblolly pine plantations

fertilized with urea in the southeast USA. Soil Sci. Soc. Am. J. 75:257–266. doi:10.2136/sssaj2010.0101.

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Figures

Figure 2.1. Location map for sites selected in the southern United States to evaluate ammonia volatilization following fertilization

with urea and enhanced efficiency fertilizers enriched with 15N after a spring and summer fertilizer application.

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Figure 2.2. Microcosm diagram for ammonia volatilization study in the southern United States selected to evaluate ammonia

volatilization following fertilization with urea and enhanced efficiency fertilizers enriched with 15N after a spring and summer

application.

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Figure 2.3. The mean percent recovery of applied 15N enriched fertilizers in microcosms, by soil depth increment, for spring and

summer application after 30 days. The >30 cm increment is immediately below the microcosm. Error bars are the standard error of

the mean.

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Figure 2.4. Cumulative mean volatilization loss from microcosms, expressed as a percent loss

of applied N fertilizer, for 6 mid-rotation loblolly pine plantations across the southern United

States after a spring (A), summer (B), and spring + summer (C) application of 15N enriched

treatments (CUF, NBPT, PCU, urea). Different letters are significant differences at α = 0.05,

and error bars are the standard error of the mean. The N application rate was 224 kg N ha-1.

A

C

B

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Tables

Table 2.1. Selected climatic and physical characteristics for sites in the southern United States selected to evaluate ammonia volatilization

following fertilization with urea and enhanced efficiency fertilizers enriched with 15N.

Site Latitude Longitude Altitude Mean Annual

Precipitation

Mean Annual

Temperature

Physiographic

Region Soil Taxonomic Class Soil Series

Soil Drainage

Class

(m) (cm) (°C)

VA 37.440087 78.662396 197 109 13 Southern Piedmont

fine, mixed, subactive, mesic Typic Hapludults

Littlejoe well

SC 33.869400 79.289300 2 125 17 Atlantic Coast

Flatwoods fine, kaolinitic, thermic

Umbric Paleaquults Byars very poorly

NC 35.317006 78.514167 0.5 121 16 Atlantic Coast

Flatwoods fine-silty, mixed, active, thermic

Typic Albaquults Leaf poorly

AR 33.422310 91.732651 69 140 16 Western Gulf Coastal Plain

fine-silty, mixed, active, thermic, Typic Glossaqualfs

Calhoun poorly

ALN 33.233371 87.232384 158 125 17 Appalachian

Plateau

loamy-skeletal, mixed, subactive, thermic, shallow,

Typic Dystrudepts Montevallo well

ALS 31.659464 86.272045 111 145 26 Southern Coastal

Plain fine, smectitic, thermic,

Typic Hapludults Arundel well

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Table 2.2. Selected characteristics of loblolly pine stands in the southern United States

selected to evaluate ammonia volatilization following fertilization with urea and enhanced

efficiency nitrogen containing fertilizers.

Site Age Trees/Hectare Height (m) DBH (cm) BA (m2ha-1)

VA 17 1100-1200 13.1 16.5 26.8

SC 16 500-600 16.0 25.7 29.7

NC 18 100-200 18.7 27.2 7.7

AR 13 300-400 15.3 20.8 22.4

ALN 19 300-400 18.8 24.2 25.2

ALS 19 500-600 18.5 21.1 20.4

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Table 2.3. Selected physical and chemical soil characteristics of loblolly pine stands in the southern United States selected to evaluate ammonia

volatilization following fertilization with urea and enhanced efficiency nitrogen fertilizers enriched with 15N.

Site Depth BD CEC pH P K Ca Mg Zn Mn Cu Fe B N C

Water Mehlich I Extractable Total

cm g

cm-3 cmol kg-1

mg kg-1 g kg-1

VA

Organic horizon

15.3 315.9

0-10 1.24 6.40 4.2 3.0 39.3 68.3 14.7 0.5 7.1 0.4 62.7 0.1 1.7 24.9

10-20 1.32 5.33 4.3 2.3 29.7 44.0 11.7 0.3 4.0 0.6 16.9 0.1 0.5 7.1

20-30 1.45 5.80 4.4 2.0 39.0 46.0 12.7 0.5 2.4 0.3 29.8 0.1 0.4 8.4

SC

Organic horizon

7.1 252.6

0-10 1.32 7.10 4.4 4.0 17.0 147.0 24.0 0.4 3.6 0.6 34.9 0.1 3.4 28.6

10-20 1.35 7.80 4.5 6.0 18.0 217.0 30.0 0.7 2.2 0.5 34.7 0.1 1.2 27.6

20-30 1.38 7.50 4.6 4.0 16.0 153.0 31.0 0.7 1.1 0.4 75.4 0.1 3.8 13.9

NC

Organic horizon

8.6 309.7

0-10 1.28 9.5 3.8 8.0 22.0 113.0 22.0 0.6 1.5 0.3 100.3 0.1 1.0 43.5

10-20 1.31 5.3 4.4 4.0 17.0 59.0 14.0 0.5 0.8 0.4 81.6 0.1 0.6 26.8

20-30 1.48 4.2 4.4 2.0 16.0 36.0 10.0 0.3 0.3 0.2 74.2 0.1 0.5 15.5

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Site Depth BD CEC pH P K Ca Mg Zn Mn Cu Fe B N C

Water Mehlich I Extractable Total

cm g

cm-3 cmol kg-1

mg kg-1 g kg-1

AR

Organic horizon

11.0 279.1

0-10 1.37 4.90 5.0 4.0 29.0 339.0 85.0 0.6 209.8 0.5 42.1 0.2 1.3 11.2

10-20 1.41 4.60 5.0 3.0 25.0 265.5 73.0 0.5 192.8 0.5 35.2 0.2 0.7 6.7

20-30 1.39 4.30 4.9 2.0 21.0 192.0 61.0 0.4 175.7 0.5 28.2 0.1 0.4 5.0

ALN

Organic horizon

9.9 224.7

0-10 1.27 5.80 5.4 3.0 45.0 449.0 57.0 1.2 126.6 0.5 17.8 0.2 1.9 21.1

10-20 1.31 4.6 5.4 2.0 44.0 421.0 71.0 0.8 81.7 0.6 13.6 0.2 0.6 6.9

20-30 1.35 4.6 5.4 2.0 44.0 408.0 65.0 0.9 117.0 0.3 14.6 0.2 0.6 4.4

ALS

Organic horizon

8.7 279.3

0-10 1.32 15.60 4.2 4.0 76.0 653.0 224.0 0.5 6.3 0.4 53.4 0.2 0.5 21.5

10-20 1.25 15.35 4.3 3.0 74.0 580.0 207.0 0.5 3.5 0.5 40.7 0.2 0.6 14.4

20-30 1.36 15.10 4.3 2.0 72.0 507.0 190.0 0.4 0.7 0.5 28.0 0.1 0.4 10.9

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Table 2.4. Mean 15N (‰) values for depth increments of microcosms 30 days after a spring and summer application of 15N enriched fertilizer

treatments (CUF, NBPT, PCU, urea) at 6 mid-rotation loblolly pine plantations across the southern United States. The nitrogen application rate

was 224 kg N ha-1. Values in parentheses are the standard error of the mean.

Control CUF NBPT PCU Urea

A) Spring

O horizon (Oi + Oe + Oa) -1.98 (0.55) 56.80 (12.90) 62.90 (13.20) 125.30 (20.80) 36.47 (3.75)

0-10 cm 3.52 (0.43) 39.88 (7.53) 34.60 (12.00) 10.71 (1.66) 23.59 (2.91)

10-20 cm 4.99 (0.58) 16.53 (4.97) 12.78 (3.10) 10.13 (1.66) 17.10 (3.45)

20-30 cm 6.48 (1.01) 15.19 (2.59) 12.61 (2.24) 10.69 (2.00) 10.95 (1.56)

> 30 cm 7.05 (0.83) 6.35 (0.78) 6.16 (0.51) 6.54 (0.98) 6.34 (0.76)

B) Summer

O horizon (Oi + Oe + Oa) -2.33 (0.70) 80.14 (8.88) 90.90 (15.40) 111.00 (12.80) 58.36 (7.68)

0-10 cm 3.32 (0.92) 30.78 (4.22) 32.27 (2.71) 38.82 (6.76) 28.42 (7.87)

10-20 cm 3.78 (0.64) 21.34 (6.37) 22.71 (8.09) 17.33 (5.96) 20.85 (7.24)

20-30 cm 5.48 (0.95) 16.80 (7.31) 13.93 (3.08) 13.53 (2.37) 9.48 (2.41)

> 30 cm 6.59 (0.74) 6.03 (0.69) 6.15 (0.70) 5.72 (0.68) 5.96 (0.55)

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Table 2.5. Results (F-values and significance) of repeated measures ANOVA testing treatment (CUF, NBPT, PCU, urea), season (spring,

summer), day (1, 15, 30), and the interactions of treatment*season, treatment*day, and treatment*season*day for NH3 volatilization loss as a

percentage of nitrogen fertilizer for sites at 6 mid-rotation loblolly pine plantations across the southern United States. The nitrogen application

rate was 224 kg N ha-1.

Effect F value Pr > F

Treatment (CUF, NBPT, PCU, urea) 58.13 <0.0001

Season (spring, summer) 2.13 0.1519

Day (1, 15, 30) 20.73 <0.0001

Treatment * Season 5.11 0.0044

Treatment * Day 1.25 0.2962

Season * Day 0.74 0.4846

Treatment * Season * Day 1.31 0.2683

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Table 2.6. P > | t | values for treatment (CUF, NBPT, PCU, urea) contrasts for mean ammonia volatilization loss for days (1, 15, 30) after

application of 15N enriched nitrogen fertilizers during two seasons (spring, summer) and combined seasons (spring + summer) at 6 midrotation

loblolly pine plantations in the southern United States. The nitrogen application rate was 224 kg N ha-1.

Spring Summer Overall (Spring+Summer)

Number of days following fertilizer application

Treatment

Contrast 1 15 30 1 15 30 1 15 30

Urea vs. CUF <0.0001 <0.0001 <0.0001 0.1119 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

Urea vs. NBPT

0.0006 0.0036 0.0033 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

Urea vs. PCU 0.0002 0.0250 0.0002 0.0003 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

CUF vs. NBPT

0.2025 0.2062 0.0287 0.3358 0.9182 0.6285 0.4007 0.5760 0.1962

CUF vs. PCU

0.3380 0.7575 0.0002 0.0866 0.8454 0.4061 0.5809 0.7105 0.4314

NBPT vs. PCU

0.7461 0.3359 0.1811 0.4635 0.9979 0.7267 0.7711 0.3540 0.0412

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Table 2.7. Correlation matrix for selected weather variables 1 day following urea

application for cumulative mean volatilization loss after application of 15N enriched nitrogen

fertilizers during two seasons (spring, summer) and combined seasons (spring + summer) at

6 mid-rotation loblolly pine plantations in the southern United States. Top values are the

Pearson Correlation and number below in parentheses represents the associated p-value.

The nitrogen application rate was 224 kg N ha-1.

Cumulative Mean Volatilization Loss (% of N

Applied)

Spring Summer Spring+Summer

Maximum Daily Temperature

0.312 (0.324)

-0.178 (0.736)

0.262 (0.216)

Minimum Daily Temperature

0.288 (0.365)

0.222 (0.673)

0.252 (0.234)

Mean Daily Temperature

0.303 (0.338)

0.036 (0.945)

0.261 (0.218)

Total Daily Precipitation

-0.512 (0.089)

N/A -0.311 (0.139)

Maximum Daily Relative Humidity

-0.248 (0.756)

0.375 (0.464)

-0.245 (0.249)

Minimum Daily Relative Humidity

0.101 (0.756)

0.375 (0.464)

-0.062 (0.772)

Mean Daily Relative Humidity

-0.152 (0.636

0.554 (0.254)

-0.249 (0.240)

Percentage of Day where Critical Relative Humidity > Relative Humidity

-0.303 (0.338)

-0.036 (0.945)

-0.261 (0.218)

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Chapter 3. Temporal Patterns of Foliar Nitrogen after Fertilization for Two Application

Seasons in Mid-Rotation Loblolly Pine Plantations (Pinus taeda L) of the

Southern United States

Abstract

The nitrogen (N) uptake of loblolly pines was determined following the surface

application of urea and three enhanced efficiency N containing fertilizers (EEFs) at six

thinned mid-rotation loblolly pine stands (Pinus taeda L.) across the southern United States.

All fertilizer treatments were labeled with 15N (~370 permil, 0.5 AP) and applied during two

different seasons (spring, summer) in 2011 to individual 100 m2 circular plots. Loblolly pine

foliage was sampled every six weeks following N fertilization over a single growing season

to assess foliar N uptake. Nitrogen fertilization increased the number of flushes and

significantly increased the individual fascicle mean mass compared to unfertilized plots over

the growing season. Nitrogen fertilization slightly increased foliar mean N concentrations

(not significant), and significantly increased individual fascicle mean N content and foliar

mean 15N (‰) over the growing season after both a spring and summer fertilization. The

proportion of individual fascicle mean fertilizer N also increased over the growing season,

with individual treatment differences gradually dissipating towards the middle to end of the

growing season. Both the cumulative N content and cumulative fertilizer N content gradually

increased across the growing season after both a spring and summer fertilization, with more

fertilizer N content in individual fascicles for the spring compared to a summer fertilization.

This research indicates N fertilization improves growth directly through N addition, but may

also make N existing naturally more readily available for plant uptake in these mid-rotation

loblolly pine plantation systems.

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3.1. Introduction

Soil nutrient deficiencies limiting the growth of forest plantations can be ameliorated

through fertilization (Ballard 1984). The amelioration of nitrogen (N) deficiencies in forest

plantations is particularly important because N is required in large quantities for foliar tissue

development, photosynthesis, and numerous biochemical processes, all which are critical to

growth (Miller 1981, Linder 1987). For example, a mid-rotation fertilization of loblolly pine

(Pinus taeda L.) plantations in the southern United States with 224 kg ha-1 N and 28 kg ha-1

phosphorous (P) have a mean growth increase of 3 m3 ha-1 over 8 years (Allen 1987, Fox et

al. 2007a). Despite fertilization improving loblolly pine growth, field trials generally show

low tree fertilizer N uptake (Mead and Pritchett 1975, Johnson and Todd 1988, Albaugh et al.

2004). In addition, some sites have no or a negative fertilizer response (Pritchett and Smith

1972, Martin et al. 1999, Amateis et al. 2000, Carlson et al. 2014) leading to a growth response

ranging from 0 m3 ha-1 yr-1 to 10 m3 ha-1 yr-1 in loblolly pine fertilizer trials (Rojas 2005, Fox et

al. 2007a). Conversely, laboratory studies for certain tree species show fertilizer N uptake

ranging from 40% to 95% (Pang 1985, Salifu and Timmer 2003, Amponsah et al. 2004).

Clearly an improved understanding of tree fertilizer N uptake and fertilizer N cycling in

forest plantations is required to properly identify sites that are responsive to fertilization.

There are several explanations for differences in tree fertilizer N uptake among studies.

Fertilizer N loss after application, primarily by ammonia (NH3) volatilization (Raymond et al.

2016), denitrification (Shrestha et al. 2014) or leaching (Binkely et al. 1995, Aust and Blinn

2004) reduces total fertilizer N entering and remaining in the system, and may reduce fertilizer

N availability for tree N uptake. Fertilizer N immobilization in ecosystem components with

varying turnover rates can make fertilizer N unavailable to crop trees for extended periods

during the rotation (Birk and Vitousek 1985, Albaugh et al. 2004, Meeks 2015). Sites

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unresponsive to N fertilization may simply have adequate plant available soil N due to high rates

of soil organic matter mineralization (Fox et al. 2007a). Finally, soil nutrient deficiencies other

than N may limit productivity (Vogel and Jokela 2011). These factors and their interactions may

explain the observed variability in fertilizer trials in southern loblolly pine systems. Addressing

these points through an improved understanding of the fertilizer N allocated to ecosystem

components, pathways and rates of fertilizer N cycling, and fertilizer N uptake in forest

plantations will improve fertilizer nitrogen use efficiency (FNUE) in these systems.

To improve fertilizer N uptake after fertilization, slow release (SRN), controlled

release (CRN) and stabilized (SNF) N fertilizers were developed (Azeem et al. 2014,

Fertilizer Institute 2015). The SRN, CRN and SNF are all enhanced efficiency fertilizers

(EEFs) (AAPFCO 2015), defined as fertilizer products that reduce nutrient loss to the

environment while increasing plant nutrient availability by slowing nutrient release or

conversion of the nutrient to forms less susceptible to losses (The Fertilizer Institute 2015).

The EEFs were developed to reduce the mechanisms of fertilizer N loss and provide a more

flexible fertilizer N application and supply with seasonal plant demand (Hauck 1985, Goertz

1993, Azeem et al. 2014). The CRN is fertilizer N that is coated or encapsulated to provide

specific rate, pattern and release durations (Chien et al. 2009). Fertilizer N in SRN is

gradually released by microbial decomposition (Trenkel 2010). Fertilizer N in SNF is treated

with inhibitors to avoid rapid N transformation to less environmentally stable forms (Shaviv

1996). Effective fertilizer N containing EEFs should have: 1) improved FNUE; 2) less N

leaching; 3) low toxicity; 4) longer N supply; 5) reduced NH3 volatilization or nitrification;

and 6) lower application cost (Allen 1984, Hauck 1985). Although EEFs were developed to

improve FNUE in agroecosystems, similar FNUE issues exist for pine plantations.

Implementing EEF technology for fertilizer N application in southern pine plantations may

improve FNUE and continue to increase the productivity of these systems. An increase in

J.E. Raymond

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ecosystem fertilizer N retention may translate to an increase in N uptake over the growing

season for loblolly pine trees.

Numerous methods have been developed to assess loblolly pine nutrient status and

evaluate potential response to fertilization (Bowen and Nambier 1984, Carter 1992). The

simplest method is visual observation (Stone 1968), but is limited because obvious symptoms

(chlorosis) can be due to multi-nutrient deficiencies (Vogel or Jokela 2011) or pathogens

(USDA 1989). Foliar analysis, including nutrient concentrations, critical nutrient levels

(Tisdale et al. 1985, Havlin et al. 2014), nutrient ratios in Diagnosis of Recommendation

Integrated System (DRIS) (Hockman and Allen 1990, Sypert 2005), and graphical analysis

(Haase and Rose 1985) improve on visual assessment, but may be of limited value if

sampling season and edaphic factors are not integrated (Carter 1992). Soil sampling and

nutrient addition experiments also provide insight but may require large sample sizes to

address soil heterogeneity (Binkely and Hart 1989). Integrated soils research has led to the

development of soil groupings based on similar parent materials within physiographic regions

that correlate with specific nutrient deficiencies, like the Cooperative Research in Forest

Fertilization (CRIFF) soil groupings in the southern United States (Jokela and Long 2000), or

groupings in the Pacific Northwest (Littke et al. 2014). Field or greenhouse bioassays using

seedlings can also be valuable, but results may be confounded by artificial laboratory

conditions and may be difficult to extrapolate to the field (Addoms 1937, Hobbs 1947,

Morrison 1974, Mead and Pritchett 1975, Birk and Vitousek 1986). The most reliable, but

expensive, assessment of nutrient deficiencies and fertilizer N uptake are fertilizer field trials

(Miller 1981, Carter 1992, Albaugh et al. 2004, Fox et al. 2007a). Stable isotopes, as with

15N, can be used to assess nutrient deficiencies and quantify fertilizer N uptake (Hauck 1968).

Fertilizers enriched with stable isotopes (15N) can trace the ultimate fate of applied N (Walker

1958, Hauck 1968, Powlson and Barraclough 1993) and be used to integrate ecosystem

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processes (Robinson 2001, Dawson et al. 2002, Templar et al. 2012). Numerous 15N tracer

studies were initially conducted in agroecosystems (Hauck and Bystrom 1971), but 15N tracer

studies have also improved the understanding of N dynamics in natural (Tietema et al. 1998,

Currie and Nadelhoffer 1999, Dinkelmeyer et al. 2003, Nadelhoffer et al. 2004, Templar et al.

2012) and plantation (Walker 1958, Hauck 1968, Mead and Pritchett 1975, Melin et al. 1983,

Chang et al. 1996, Bubb et al. 1999, Mead et al. 2008, Werner 2013) forests. Often several

methods need to be combined at sites to improve the understanding of nutrient status and

fertilizer responsiveness (Miller 1981, Birk and Vitousek 1985, Sypert 2005).

All fertilizer N in this study, both urea and EEFs, were enriched with 15N (~370‰, 0.5

AP) to improve the understanding of N uptake of loblolly pines in the southern United States

over the course of a single growing season. The statistical hypotheses for this research are:

HO1: There are no differences in foliar growth among treatments.

HO2: There are no differences in foliar N uptake among treatments.

HO3: There are no differences in foliar fertilizer N uptake among treatments.

3.2. Methods and Materials

3.2.1. Experimental Design and Site Description

This experiment used a split plot complete block design to test temporal differences in

fertilizer N uptake of loblolly pine. Five fertilizer treatments (main plots) were applied at two

different seasons, spring versus summer (split plot) in mid-rotation loblolly pine plantations.

The fertilizer treatments were the main plots with a single replication of each treatment

combination (fertilizer source and season of application) at each individual site. The six sites

served as blocks and provided replication. The split plot was the application date (spring,

summer). The six sites were selected from an existing network of forest thinning and

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fertilization studies in mid-rotation loblolly pine plantations across the South (Figure 3.1).

Selected site characteristics are detailed in Table 3.1 and Table 3.2.

Twelve 100 m2 circular plots were installed at the six sites in early March 2011. At

each plot center were two co-dominant loblolly pine trees with similar height (HT) and

diameter (DBH). Six plots were used for the spring treatments and the other six plots were

used for the summer treatment. Prior to treatments, the DBH and HT were measured for all

trees within plots after installation but prior to treatment application.


Five fertilizer treatments were applied at the two application times: 1) urea; 2) urea

impregnated with N-(n-Butyl) thiophosphoric triamide (NBPT); 3) urea impregnated with

NBPT and coated with monoammonium phosphate (CUF); 4) urea coated with a polymer

(PCU); and a 5) control treatment where no fertilizer N was added. Urea (46-0-0) was used

because it is the most commonly applied fertilizer N source used in southern forests (Fox et

al. 2007a). The enhanced efficiency fertilizers (EEFs) tested in this study were developed to

reduce NH3 volatilization and release N gradually to the environment. For the NBPT

treatment, urea (46-0-0) granules were impregnated with N-(N-butyl) thiophosphoric triamide

at 26.7% by weight to inhibit urease activity. For the CUF treatment (39-9-0), urea granules

were impregnated with NBPT and then coated with an aqueous boron and copper sulfate

binder solution to slow N release to the environment. A monoammonium phosphate coating

was then added for P. The PCU (44-0-0) treatment had polymer coatings covering urea

granules with pores in the coating to slowly release N (~80%) over 120 days. The application

rate for all treatments was 224 kg N ha-1. Because CUF contained P, P was added to the other

fertilizer treatment at the equivalent rate of 28 kg P ha-1 using triple superphosphate (TSP).

The urea used as the base for all treatments was enriched with the stable isotope 15N (~370‰,

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0.5 AP) (Nadelhoffer and Frye 1994). All fertilizer treatments were broadcast applied by

hand to the forest floor in individual 100m2 circular plots described above. Individual

fertilizer N treatments were applied to the individual 100m2 circular plots on a single day for

each application season at each individual site.

3.2.3. Experimental Method- Field Sampling

Foliage was sampled from the upper 1/3rd of the canopy from both central trees

immediately prior to individual fertilizer N treatment application and represented “week 0”.

The spring application date was from March 26 to April 8 2011, and the summer application

was from June 18 to June 30 2011. Sampling of foliage from the upper 1/3rd of the canopy

from both central trees for all individual plots occurred every 6 weeks after the fertilizer

treatments were applied until the dormant season. There were 6 sampling periods (week 0, 6,

12, 18, 24, 30) for spring fertilization and 4 sampling periods (week 0, 6, 12, 18) for summer

fertilization at each site. Foliage was separated by each successive individual vegetative

development over the growing season, termed flush, for each sampling period (Figure 3.2).

3.2.4. Experimental Method - Laboratory Processing

The foliage samples were oven dried in a forced air oven with a constant temperature

of 60°C for 7 days. After oven drying, 25 intact and undamaged foliar fascicles were

weighed. After weighing, the 25 fascicles from the sample were ground to a fine powder in a

ball mill (Retsch® Mixer Mill MM 200, Haan, Germany) for 1 minute at 25 revolutions per

second (rps). The ball mill was cleaned with an ethanol solution and allowed to dry to reduce

contamination after the processing of each sample. Two milligrams (+0.20) of ground foliage

were weighted on a Mettler-Toledo© MX5 microbalance (Mettler-Toledo, Inc., Columbus,

OH, USA) and placed in separate tin capsules. Samples were analyzed to determine the

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15N/14N isotope ratio and total N of the sample on a coupled elemental analysis-isotope ratio

mass spectrometer (IsoPrime 100 EA-IRMS, Isoprime© Ltd., Manchester, UK) at the Forest

Soils and Plant Nutrition Laboratory at Virginia Polytechnic Institute and State University.

3.2.5. Calculations

Individual fascicle N content was calculated by multiplying individual fascicle mass (g) by N

concentration (Equation 3.3). Individual fascicle fertilizer N content (Equation 3.1 to

Equations 3.5) was calculated using individual fascicle mass, N concentration, and 15N (‰)

(Powlson and Barraclough 1993, Nadelhoffer and Fry 1994, Nadelhoffer et al. 1995). The

uptake of N from sources existing in the system not originating from the fertilizer was

calculated in Equation 3.6.

[3.1] %Fertilizer N in individual fascicles: ((δ15Nfinal-δ15Ninitial)/(δ15Nlabeled-δ15Ninitial))*(100)

[3.2] %15N in individual fascicles from labeled treatment: ((δ15Ncomponent-δ15Npretreatment) /

(δ15Nfertlizer- δ15Npretreatment)) * (100)

[3.3] Total N (g) = (Individual fascicle dry weight * %N) / (100)

[3.4] Fertilizer N (g) in individual fascicle = (Total N * %15N from fertilizer) / (100) )

[3.5] %N originated from fertilizer in individual fascicle =

[(N originated from fertilizer (g) in individual fascicle) / (15N fertilizer (g) ) ] * (100)

[3.6] %N originated from native N sources in individual fascicle =

(Total N in individual fascicle) – (%N originated from fertilizer in individual fascicle)


Individual fascicle mean mass, foliar N mean concentration, individual fascicle mean N

content, mean foliar 15N, and individual fascicle mean fertilizer N content was analyzed using

a repeated measure analysis of variance (ANOVA) using PROC MIXED in SAS® 9.4 (SAS

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Institute, Cary, NC). Several alternative spatial covariate structures were tested, and the

optimal structure was the unstructured (UN) covariate structure based on Akaike’s

Information Criteria (AIC). Individual fascicle mean mass, mean foliar N concentration,

individual fascicle mean N content, mean foliar 15N, and individual fascicle mean fertilizer N

content were the response variables for the model, sampling period (weeks 0, 6, 12, 18, 24,

30) and flush (0, 1, 2, 3) were repeated measures, fertilizer treatment (CUF, NBPT, PCU,

urea, control) and season (spring, summer) were fixed effects, and site (VA, ALN, AR, ALS,

SC, NC) was a random effect. (Littell et al. 2006). Levels of significance were set at α = 0.05

and the P > |t| values for the treatment means were tested using the SLICE function in PROC

MIXED. The post-hoc analysis used was Tukey’s HSD.

3.3. Results

3.3.1. Flush Development

As the growing season progressed, a larger number of flushes occurred in fertilized

plots compared to control plots after both the spring and summer fertilization (Table 3.3).

Twelve weeks after the spring fertilization, all fertilized treatments had developed a

2nd flush (flush 2) at all 6 study sites while the control treatment had a single site with a flush

2. At the end of the growing season, 30 weeks after the spring fertilizer application, all

fertilized plots had developed a 3rd flush (flush 3) at all 6 study sites while the control

treatment had a single study site with a flush 3.

A similar trend occurred following the summer fertilizer application (Table 3.3.) At

the end of the growing season, 18 weeks after the summer fertilizer application, fertilizer

treatments at three of the study sites had produced a flush 3 but only one site had produced a

flush 3 for the control treatment.

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3.3.2. Individual Fascicle Mass

Eighteen weeks after a spring fertilization and 6 weeks after a summer fertilization,

most individual fertilizer treatments had a greater mean individual fascicle mass (g)

compared to the control for flush 1 and flush 2 (Table 3.4). Differences did not exist for the

mean individual fascicle mass for flush 0 or flush 3 after either a spring or summer

fertilization.

After a spring fertilization, the mean individual fascicle mass (g) for flush 0 and flush

1 in week 6 and week 12 for the control ranged from 0.14 g to 0.19 g, compared to the range

of individual fertilizer treatments which ranged from 0.14 g to 0.21 g (ns). The mean

individual fascicle mass for flush 1 in week 18 for all fertilizer treatments was 0.24 g

compared to the 0.17 g for the control, and in flush 2 both CUF and urea (0.22 g) were

greater than the control (0.17 g). By week 24 the mean individual fascicle mass for flush 1

was greater for both CUF (0.25 g) and PCU (0.23 g) compared to the control (0.18 g) while in

flush 2 both CUF (0.21 g) and NBPT (0.21 g) were greater than the control (0.15 g). At the

end of the growing season the mean individual fascicle mass for flush 1 for CUF (0.23 g),

NBPT (0.25 g), PCU (0.24 g), and urea (0.24 g) was greater than the control (0.17 g), and in

flush 2 CUF (0.25 g), NBPT (0.22 g) and urea (0.23 g) were greater than the control (0.17 g)

while CUF was greater than PCU (0.19 g).

For the summer fertilization, mean individual fascicle mass (g) for flush 0 for all

sampling periods ranged from 0.18 g to 0.20 g for the control and 0.17 g to 0.24 g for all

individual fertilizer treatments (ns) (Table 3.4). The mean individual fascicle mass in week 6

for flush 1 and flush 2 was greater for NBPT (0.25 g, 0.23 g respectively), PCU (0.23 g, 0.20

g respectively), urea (0.24 g, 0.20 g respectively), and for CUF (0.22 g) only in flush 2,

compared to the control (0.20 g, 0.16 g respectively). In week 12, the mean individual

fascicle mass was greater for flush 1 and flush 2 for NBPT (0.23 g, 0.23 g respectively) and

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urea (0.24 g, 0.23 g respectively) compared to the control (0.18 g, 0.17 g respectively). By the

end of the growing season, mean individual fascicle mass for flush 1 was greater for CUF

(0.24 g), PCU (0.24 g) and urea (0.26 g) compared to the control (0.20 g), and both NBPT

(0.24 g) and urea (0.25 g) were greater than the control (0.18 g) in flush 2.

3.3.3. Foliar N Concentration

The mean foliar N concentration (g kg-1) increased slightly following both a spring

and summer fertilization for all individual fertilizer treatments compared to the control,

although most differences were not significant (ns) (Table 3.5; In Appendix: Figure A.3.1.,

Table A.3.1, Table A.3.3, Table A.3.5., Table A.3.8.).

Eighteen weeks after the spring fertilization, there was a significant increase in the

mean foliar N concentration of flush 2 between NBPT (16.0 g kg-1) and the control (11.4 g

kg-1). By week 30, the mean foliar N concentration of both NBPT (13.0 g kg-1) and urea (12.6

g kg-1) were greater than the control (8.6 g kg-1) in flush 0.

Twelve weeks after summer fertilizer application, urea (14.2 g kg-1) was greater than

the control (10.4 g kg-1) in flush 2. All other treatment differences for mean foliar N

concentrations for individual flushes during each sampling period were not significant (ns).

3.3.4. Individual Fascicle N Content

The mean individual fascicle N content (mg N fascicle-1) was greater for most

individual treatments compared to the control 18 weeks after a spring fertilization and 6

weeks after the summer fertilization (Table 3.6; In Appendix: Table A.3.6., Table A.3.9.).

After the spring fertilization, the mean individual fascicle N content (mg) for all

flushes in week 6 and week 12 ranged from 0.67 mg to 1.95 mg for the control compared to

1.43 mg to 2.38 mg for all individual fertilizer treatments (ns) (Table 3.6). There were no

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differences in mean individual fascicle N content in flush 0 among treatments for any

sampling period except in week 30 where NBPT (2.72 mg) was greater than the control (1.74

mg). By week 18, differences occurred between individual fertilizer treatments and the

control for the mean individual fascicle N content, where CUF (3.35 mg), NBPT (3.57 mg),

PCU (3.43 mg) and urea (3.44 mg) were greater than the control (1.93 mg) in flush 1, and

CUF (2.84 mg), NBPT (3.29 mg) and urea (3.08 mg) were greater than the control (1.82 mg)

in flush 2. In week 24, all individual fertilizer treatments were greater in flush 1 (range: 2.86

mg to 3.45 mg) and in flush 2 (range: 2.43 mg to 3.00 mg) than the control (2.02 mg, 1.59 mg

respectively). By week 30, all individual fertilizer treatments were greater than the control for

both flush 1 and flush 2, but NBPT (3.62 mg) was greater than CUF (2.72 mg) in flush 1 and

CUF (3.51 mg) was greater than PCU (2.75 mg) in flush 2.

For the summer fertilization, mean individual fascicle N content (mg) for flush 0 in

week 6 and week 12 was greater for NBPT (2.58 mg, 2.64 respectively) compared to the

control (1.99 mg, 1.98 mg respectively) (Table 3.6). Six weeks after the summer fertilization,

all fertilizer treatments had a greater mean individual fascicle N content in both flush 1

(range: 2.94 mg to 3.45 mg) and flush 2 (range: 2.39 mg to 3.11 mg) than the control (2.25

mg, 1.78 mg respectively), and both CUF (2.98 mg) and NBPT (3.11 mg) were greater than

PCU (2.39 mg) in flush 2. By week 12, all fertilizer treatments were greater in flush 1 (range:

2.68 mg to 3.26 mg) and flush 2 (range: 2.71 mg to 3.23 mg) than the control (1.77 mg, 1.18

mg respectively) with no differences among individual treatments. At the end of the growing

season, CUF (3.19 mg) and urea (3.46 mg) were greater than the control (2.15 mg) in flush 1,

all fertilizer treatments were greater in flush 2 (range: 2.46 mg to 3.46 mg) than the control

(2.11 mg), and both PCU (2.78 mg) and urea (3.46 mg) greater than CUF (2.46 mg) in flush

2.

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3.3.5. Foliar 15N

The mean foliar 15N (‰) was different for most fertilizer treatments for flush 0 and

flush 1, and all fertilizer treatments for flush 2, compared to the control for all sampling

periods after a spring fertilization (Table 3.7). After the summer fertilization, all individual

fertilizer treatments were different compared to the control for all flushes during each

sampling period beginning 12 weeks after a summer fertilization (Table 3.7; In Appendix

Figure A.3.2., Table A.3.2., Table A.3.4., Table A.3.5., Table A.3.8.).

Across the entire growing season after a spring fertilization, the control mean foliar

15N ranged from -3.43‰ to -1.56‰ for all flushes (Table 3.7). There were differences in

flush 0 between the control and CUF (58.8‰) in week 12, in week 18 and week 24 between

the control (-3.43‰, -3.38‰ respectively) and CUF (69.9‰, 56.7‰ respectively), NBPT

(57.7‰. 56.3‰ respectively) and urea (60.4‰, 65.9‰ respectively), and the control (-

2.50‰) and all individual fertilizer treatments (range: 53.5‰ to 80.1‰) by week 30. In week

6 and week 12, only PCU (17.8‰, 40.9‰ respectively) was not different from the control in

flush 1. After 18 weeks, mean foliar 15N (‰) of all individual fertilizer treatments for flushes

1, 2, and 3 ranged 73.8‰ to 134.2‰.

Six weeks after the summer fertilization there were differences in the mean foliar 15N

(‰) between the control (-2.90‰ to -2.10‰) and all fertilizer treatments (range: 52.1‰ to

72.8‰) except in flush 1 and flush 2 for PCU (22.9‰, 28.6‰ respectively). Twelve weeks

after the summer fertilization, there were differences between the control (range: -3.04‰ to -

2.55‰) and all individual fertilizer treatments (range: 40.5‰ to 102.6‰).

3.3.6. Individual Fascicle Fertilizer N Content

The mean individual fascicle fertilizer N content (mg N fascicle-1) generally increased

in all flushes across the entire growing season for all individual fertilizer treatments after both

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the spring and summer fertilization (Table 3.8; In Appendix Table A.3.6., Table A.3.9.).

Because no fertilizer was applied to the control treatment, the value for all flushes for all

sampling periods was 0.00 mg of fertilizer N.

Six weeks after the spring fertilization, the mean individual fascicle fertilizer N

content of flush 0 for all treatments ranged from 0.00 mg to 0.19 mg (Table 3.8). In flush 1 of

week 6, CUF (0.29 mg), NBPT (0.57 mg) and urea (0.33 mg) were greater than both the

control and PCU (0.07 mg), and NBPT was greater than both CUF and urea. In week 12,

CUF (0.25 mg) was greater than PCU (0.11 mg) in flush 0, while both NBPT (0.43 mg) and

urea (0.39 mg) were greater than PCU (0.11 mg) in flush 1. In week 18 no differences

occurred among individual fertilizer treatments for any flush, and mean individual fascicle

fertilizer N content ranged from 0.25 mg to 0.90 mg. By week 24, the mean individual

fascicle fertilizer N content for NBPT (0.87 mg) was greater than PCU (0.49 mg) in flush 2.

At the end of the growing season NBPT (0.84 mg) was greater than PCU (0.54 mg) in flush

1, with no other differences among individual fertilizer treatments for mean individual

fascicle fertilizer N content.

After the summer fertilization, the mean individual fascicle fertilizer N content for

individual fertilizer treatments increased for all flushes for all sampling periods until the end

of the growing season (Table 3.8). In week 6 for flush 2, CUF, NBPT and urea (range: 0.44

mg to 0.50 mg) were greater than PCU (0.16 mg). In week 12, the mean individual fascicle

fertilizer N content increased for all treatments and ranged from 0.21 mg to 0.78 mg in

flushes 0, 1, and 2. Both NBPT (0.78 mg) and urea (0.65 mg) also had greater mean

individual fascicle fertilizer N content than both CUF (0.45 mg) and PCU (0.43 mg) in flush

2 of week 12. By week 30, the mean individual fascicle fertilizer N content was greater for

urea (0.65 mg) than CUF (0.32 mg), NBPT (0.35 mg) and PCU (0.19 mg) in flush 0. The

mean individual fascicle fertilizer N content for both urea (0.57 mg) and CUF (0.54 mg) were

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greater than PCU (0.33) in flush 1, and in flush 2 CUF (0.58 mg), NBPT (0.74 mg) and urea

(0.70 mg) were all greater than PCU (0.40 mg).

3.3.7. Cumulative Individual Fascicle N Content

The cumulative individual fascicle N content (N mg fascicle-1) increased for all

treatments across the growing season, with differences between all individual fertilizer

treatments and the control occurring by week 18 after the spring fertilization and week 6 after

a summer fertilization (Figure 3.3; In Appendix Table A.3.6., Table A.3.9.).

After the spring fertilization, the cumulative individual fascicle foliar N content was

similar in week 6 and week 12 between the control (3.34 mg, 4.11 mg respectively) and all

individual fertilizer treatments (range: 4.17 mg to 4.88 mg, 4.11 mg to 5.07 mg respectively)

(Figure 3.3). By week 18 the cumulative individual fascicle foliar N content for all individual

fertilizer treatments (range: 8.04 mg to 9.33 mg) was greater than the control (5.77 mg).

Differences for the cumulative individual fascicle foliar N content continued through the

remainder of the growing season with individual fertilizer treatments (range: 10.50 mg to

12.48 mg) consistently greater than the control (range: 7.47 mg to 7.54 mg).

For the summer fertilization, with the cumulative individual fascicle foliar N content

for all individual fertilizer treatments (range: 7.86 mg to 9.14 mg) was greater compared to

the control (6.02 mg) 6 weeks after fertilization. By week 12, the cumulative individual

fascicle foliar N content for individual fertilizer treatments increased to a range of 9.65 mg to

11.48 mg compared to 5.93 mg for the control which decreased from the previous sampling

period. By the end of the growing season, the cumulative individual fascicle foliar N content

of the control had increased to 8.52 mg but was still lower compared to all individual

fertilizer treatments (range: 10.19 mg to 12.95 mg).

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3.3.8. Total Cumulative Individual Fascicle Mean Fertilizer N Content

The total cumulative individual fascicle mean fertilizer N content (fertilizer N

mg/fascicle) increased for all individual treatments across the entire growing season in all

sampling periods after both the spring and summer fertilization (Figure 3.4; In Appendix

Table A.3.7.).

For the sampling weeks 6, 12, and 18 after a spring fertilization, the total cumulative

individual fascicle mean fertilizer N content for the PCU treatment was lower (0.13 mg, 0.40

mg, 1.47 mg respectively) than CUF (0.62 mg, 0.87 mg, 2.13 mg respectively), NBPT (0.79

mg, 0.87 mg, 2.0 mg respectively) and urea (0.58 mg, 0.71 mg, 1.87 mg respectively) (Figure

3.4). For week 24 and week 30, the total cumulative individual fascicle mean fertilizer N

content was similar among all individual fertilizer treatments, ranging from 2.20 mg to 2.54

mg for week 24 and 2.41 mg to 3.02 mg in week 30.

For summer fertilization, the total cumulative individual fascicle mean fertilizer N

content increased over the growing season (Figure 3.4). For all weeks sampled after

fertilization (6, 12, 18), the PCU treatment (0.46 mg, 1.20 mg, 1.51. mg respectively) was

generally lower than the other fertilizer treatments including CUF (1.18 mg, 1.35 mg, 2.02

mg respectively), NBPT (1.15 mg, 1.96 mg, 2.50 mg respectively) and urea (1.09 mg, 1.70

mg, 2.41 mg respectively).

3.4. Discussion

Our study compared the temporal trends of foliar N uptake of loblolly pines with five

different treatments for two different application seasons across the southern United States.

Our study tested three primary hypotheses: 1) no differences existed for foliar growth and

development among treatments; 2) differences did not occur in foliar N uptake among

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treatments; and 3) that there were no differences in foliar fertilizer N uptake among fertilizer

treatments.

Our first hypothesis that differences did not exist in foliar growth and development

among treatments (control, CUF, NBPT, PCU, urea) was rejected. Differences in foliar

growth, assessed by differences in the development and frequency of flushes and individual

fascicle mass, occurred for all individual fertilizer treatments compared to the control after

both a spring and summer fertilization. Loblolly pine shoot and foliar development, termed

flushes or cohorts (Figure 3.2), are episodic growth events that increase in frequency under

favorable conditions (Miller 1966, Shultz 1997). The first flush of the growing season, flush

1, generally has the largest shoot elongation, individual fascicle mass, needle length and

foliar N of any flush during the growing season (Griffing and Elam 1971, Boyer and South

1989, Zhang et al. 1997). Improving soil nutrition through the application of N + P fertilizers

generally increases flush development and individual fascicle mass (Adams and Allen 1985,

Valentine and Allen 1990, Zhang et al. 1997, Albaugh et al. 1998, 2004, Tang et al. 1999,

Carlson et al. 2014) for loblolly pines.

In our study, fertilizer treatments increased the frequency and development of both

flushes and individual fascicle mass when compared to the control, although differences

among treatments were difficult to distinguish. For the spring fertilization, all fertilizer plots

at all research sites developed a 3rd flush, compared to one control plot at one site in SC. The

SC site was one of the more productive sites in this study due to high amounts of

decomposing soil organic matter and this site is likely not N limited. Additionally, flush

development in this study was more rapid over the growing season compared to the control.

Although an increase in flushes after the summer fertilization also occurred, differences in

flush frequency were not as great as with the spring fertilization. For the summer fertilization,

only the control site at SC had developed a 3rd flush. Additional flushes and increased

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cumulative foliar mass in the fertilized plots leads to expansion of leaf area (Miller 1981,

Vose and Allen 1988, Albaugh et al. 1998, Fox et al. 2007a) increases light interception, and

directly improves stand productivity (Russell et al. 1989, Cannell 1989).

Our second hypothesis that foliar N uptake among treatments did not differ was also

rejected. Although foliar N concentrations were generally not significantly different between

the control and individual fertilizer treatments for the spring or summer fertilization, there

was an increasing trend for all fertilizer treatments compared to the control through the

growing season and is similar to the findings of others (Switzer et al. 1966, Wells et al. 1975,

Zhang and Allen 1996, Albaugh et al. 1998, Gough et al. 2004, Carlson et al. 2014).

Significant differences in foliar N concentration can be muted due to dilution effects from

increased biomass production in fertilized plots (Jose et al. 2003) but can be accounted for

through calculating the individual fascicle N content. Once individual fascicle N content was

calculated, there were significant differences between most fertilizer treatments and the

control and was similar to the findings of others (Smith et al. 1970, 1971, Switzer et al. 1972,

Murthy et al. 1996, Albaugh et al. 2004, Choi et al. 2005, Carlson et al. 2014). Additionally,

more cumulative individual fascicle N uptake occurred with all fertilizer treatments compared

to unfertilized treatments.

One additional point is the foliar N concentrations in flush 0 (foliage produced in the

previous growing season) for the control was consistently low and gradually decreased

through the growing season for both spring and summer sampling compared to fertilized

treatments, indicating retranslocation of N prior to senescence. Photosynthesis is greatly

reduced when foliar N concentrations go below 0.8% (Gough et al. 2004), a value approached

for in flush 0 of the control treatments. Conversely, the flush 0 for all individual fertilizer

treatments remained at or significantly above 1.00% for the entire growing season for both

the spring and summer fertilization. This finding indicates a potential for flush 0 to extend

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photosynthesis for a period of time near the end of the growing season for fertilized sites,

possibly increasing productivity.

The third hypothesis that the foliar fertilizer N uptake was the same among individual

fertilizer N treatments was also rejected. Using N containing fertilizers enriched with the

stable isotope 15N can improve the understanding of the fate of applied N in forest systems

and distinguish fertilizer N uptake in trees compared to the uptake of the N natural existing in

the system (Walker 1958, Pritchett and Smith 1972, Nadelhoffer and Fry 1994, Templar et al.

2012). Our study found consistent increases in the foliar 15N (‰) signature for all fertilizer

treatments across the entire growing season after both a spring and summer fertilization

which continued across the entire growing season for all fertilizer N treatments. There were

also slight differences among fertilizer treatments for certain sampling weeks following both

a spring and summer fertilization. Specifically, CUF, NBPT and urea all had greater foliar

15N values compared to both the control and PCU earlier in the sampling period, although the

differences between foliar 15N values among all fertilizer treatments generally dissipated near

the middle of the growing season. One explanation for this finding relates to the release

mechanisms and patterns for of each fertilizer treatment.

The EEF treatments used in this study had inhibitors, coatings or a combination to

reduce fertilizer N losses through ammonia (NH3) volatilization and provide a more gradual,

constant release of fertilizer N over the growing season (Hauck 1985, Goertz 1993, Azeem et

al. 2014, Havlin et al. 2014). Large fertilizer N losses can occur via the ammonia (NH3)

volatilization pathway immediately following urea fertilization, with losses highly dependent

on the interaction of environmental and edaphic factors (Cabrera et al. 2005, Zerpa and Fox

2011, Elliot and Fox 2014, Raymond et al. 2016). Coatings or chemical additives applied to

fertilizer N (urea) may require degradation that are dependent on the interaction of weather

and edaphic factors prior to the fertilizer N becoming plant available, creating a lag period

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between fertilizer N application and plant N availability. This release lag extends the period

when fertilizer N is plant available further into the growing season compared to urea,

providing a potentially more synchronous N supply to plant N demand.

Urea, the most commonly applied form of fertilizer N in southern forestry due to its

lowest expense per unit N (Allen 1987, Fox et al. 2007a), rapidly degrades (urea dissolution)

after application and provides N immediately to plants and soil microbes (Hauck and

Bremner 1965). Both urea and urea treated with NBPT likely had the quickest fertilizer N

release because there were no coatings to degrade. Evidence is provided in this study where

certain flushes for both urea and NBPT have significantly greater individual fascicle mean

fertilizer N compared to CUF, but more frequently with PCU. The CUF treatment had urea

granules impregnated with NBPT to reduce urease activity with an exterior water soluble

coating requiring degradation prior to fertilizer N release to the environment. The combined

technologies in CUF assist in reducing NH3 volatilization compared to urea (Raymond et al.

2016) and provide a more gradual release of fertilizer N to the environment. A lag in fertilizer

N release for CUF is supported in this study by the foliar 15N data where CUF, specifically

earlier in the growing season for certain flushes, had lower foliar 15N values compared to both

NBPT and urea. Yet most of these differences disappeared as the growing season progressed.

Research in agriculture systems has shown it can take 2 days to 21 days for complete

hydrolysis of soluble fertilizers (NBPT, CUF and urea) and is highly dependent on

environmental variables during this period such as relative humidity, precipitation, and

temperature (Gould et al. 1973, Bayrakli 1990, Shaviv 1996, Azeem et al. 2014, Havlin et al.

2014).

Conversely, the PCU technology is a semi-permeable coating surrounding urea

granules which requires water to enter the granule, urea dissolution to occur, and a threshold

internal pressure in the granule to force the fertilizer N in solution out of the granule and into

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the environment. This technology likely provides the most extended, gradual release of

fertilizer N to the environment with estimates of fertilizer N release ranging from 3 months to

6 months, or potentially longer, and dependent primarily on moisture and temperature

(Shaviv 1996, Azeem 2014). Foliar 15N data from this study again supports this statement,

with PCU generally having the lowest foliar 15N values compared to all other individual

treatments early in the growing season.

Although fertilizer release patterns have been shown to be generally dependent on a

complex interaction of environmental variables in agriculture, less is known about release

patterns of EEFs and how this translates to fertilizer N uptake for trees in forested systems.

One of the few studies examining EEF release patterns in loblolly pine plantation systems for

these same products is by Werner (2013). Werner (2013) observed an increase in NH4+ using

ion exchange resins (IER) 14 days after a fall application for CUF, NBPT and urea with no

differences among these treatments, compared to an increase in NH4+ for PCU at 49 days

after application. Werner (2013) also found a higher percentage of labeled 15N fertilizer in

certain components of the tree for CUF, NBPT and urea when compared to PCU, indicating a

correlation between release patterns and fertilizer N uptake by loblolly pines. Results from

our study indicate similar trends for the respective fertilizer treatments for both a spring and

summer fertilizer application. Few differences occurred among fertilizer treatments early in

the growing season, these differences gradually fade. Additional research will be needed to

understand release patterns in forested systems, and how much of these products remain in

the soil to extend potential plant N availability for successive growing seasons.

A final point of this research is the overall foliar N uptake of the loblolly pines.

Although it is evident that fertilizer N additions increase overall foliar N uptake, it does not

account for all of the additional foliar N uptake. If fertilizer N uptake had accounted for all

the additional N in the foliage for the fertilizer treatments, the sum of the cumulative fertilizer

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N for individual treatments in fascicles and the control (unfertilized) individual fascicle N

would equal the cumulative individual fascicle N content for each fertilizer treatment- but this

did not occur. For example, at the end of the growing season the individual fascicle mean

fertilizer N content was approximately 3.00 mg (CUF = 2.78 mg, NBPT = 3.02 mg, PCU =

2.41 mg, urea = 2.98 mg). Yet when the cumulative individual fascicle N content for the

control plot (7.47 mg) is added to individual fascicle mean fertilizer N content values for the

individual fertilizer treatments, there is a discrepancy of approximately 2.50 mg (CUF =

12.26 mg, NBPT = 12.48 mg, PCU = 11.48 mg) for the EEFs and less than 0.50 mg for urea

(10.77 mg). Additional research will be required to understand the mechanisms behind this

process, but one hypothesis is that the enhanced efficiency fertilizers specifically retain more

fertilizer N in the soil compared to urea (Raymond et al. 2016). This would translate to an

increase in fertilizer N availability to the loblolly pines and also an increase in the availability

of naturally existing N.

Fertilization research in southern loblolly pine plantations has improved the

productivity of these systems (Albaugh et al. 1998, Vogel and Jokela 2011, Carlson et al.

2014), but understanding mechanisms that quantify the proportion of fertilizer N taken up by

loblolly pines is generally hampered by the inability to distinguish the origins of natural

versus fertilizer N. Fertilizer use efficiency is usually assessed through ecosystem

productivity metrics such as foliar N concentrations, foliar nutrient ratios, diameter and

height measurements, leaf area index, root to shoot ratios, or various other metrics. By using

fertilizers enriched with the stable isotope 15N, this research has been able to improve the

understanding of plant uptake and integrate processes, mechanisms and pathways of the

fertilizer N cycle in southern loblolly pine plantations.

This research has refined the understanding of foliar N uptake over the course of a

growing season after a spring and summer N application in mid-rotation loblolly pines

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through the use of fertilizers enriched with the stable isotopes 15N. Representative sites were

chosen across an extensive geographic region where loblolly pine plantations are

operationally fertilized and intensively managed to integrate the range of climatic and

edaphic variables occurring in the southern United States. The synthesis of significant factors

from this study help support and add to the knowledge obtained from the numerous other

studies in forest ecosystems focused on smaller regions. The results from this study will

continue to improve our ability to assess loblolly pine fertilizer N response through the

refinement of fertilizer use efficiency for these high production systems.

3.5. Conclusions

The primary results from this research specific to loblolly pines were: 1) significantly

more foliar growth occurs at a faster rate after N fertilization; 2) N fertilization increases N

uptake in all stages of foliar development, including older foliage, which increases the N

concentration of individual fascicles and increases photosynthetic potential of older foliage;

and 3) significantly more foliar N uptake occurs after N fertilization of both fertilizer and

naturally existing N. In a companion study, Raymond et al. (2016) found NH3 volatilization

was reduced with the same EEFs used in this study to assess foliar N uptake. Additional

research will be required to determine if the additional fertilizer N remaining in the soil from

the EEFs will translate into additional plant available N in successive seasons and continue to

increase fertilizer N use efficiency. Results from this research can be applied through the

southern United States where mid-rotation loblolly pine are intensively managed and should

assist in improving forest managers goals of increasing N use efficiency to increase

productivity.

105

Acknowledgments


National Science Foundation Center for Advanced Forest Systems (CAFS) and the United

States Department of Agriculture National Institute of Food and Agriculture McIntire-Stennis

Capacity Grant. We thank David Mitchem for assistance in the laboratory, Andy Laviner for

field assistance and the numerous work study and seasonal student employees for both field

and laboratory assistance.

106

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plantations fertilized with urea in the southeast USA. Soil Sci. Soc. Am. J. 75:257–266. doi:10.2136/sssaj2010.0 101.

Zhang, S., and H.L. Allen. 1996. Foliar nutrient dynamics of 11-year-old loblolly pine in response to nitrogen fertilization. Can. J. For. Res. 26: 1426–1439.

Zhang, S., H.L. Allen., and P.M. Dougherty. 1997. Shoot and foliage growth phenology of

loblolly pine trees as affected by nitrogen fertilization. Can.J. For. Res. 27(9): 1420- 1426.

114

Figures

Figure 3.1. Location map of loblolly pine stands in the southern United States selected to

evaluate temporal foliar uptake of fertilizer nitrogen during the growing season using 15N

enriched fertilizers following the application of urea or enhanced efficiency nitrogen

fertilizers after a spring and summer fertilizer application.

1

115

Figure 3.2. Representation of individual flush growth on a loblolly pine sampled every six

weeks over the growing season after a spring and summer fertilization in the southern United

States selected to evaluate N uptake of urea or enhanced efficiency N containing fertilizers.

116

Figure 3.3. Cumulative individual fascicle N content (mg N fascicle-1) of individual flushes

combined for sampling weeks (6, 12, 18, 24, 30) after spring and summer fertilization (2011) for

loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea

or enhanced efficiency N containing fertilizers enriched with 15N. The N application rate was

224 kg N ha-1. N = 6.

Spring

Spring Summer

Control Control

CUF CUF

117

Figure 3.3 (cont.)

Spring Summer

PCU PCU

NBPT NBPT

Urea Urea

118

Figure 3.4. Cumulative individual fascicle N content from fertilizer (mg N fascicle-1) of

individual flushes (0, 1, 2) combined for sampling weeks (6, 12, 18) after spring and summer

fertilization (2011) for loblolly pine stands in the southern United States selected to evaluate

fertilizer N uptake of urea or enhanced efficiency N containing fertilizers enriched with 15N. The

N application rate was 224 kg N ha-1. N = 6.

Spring Summer

CUF CUF

NBPT NBPT

J.E. Raymond

119

Figure 3.4. (continued)

Spring Summer

PCU PCU

Urea Urea

120

1

121

Tables

Table 3.1. Selected climate and physical characteristics of loblolly pine stands in the southern United States selected to evaluate temporal foliar

uptake of fertilizer nitrogen following the application of urea or enhanced efficiency nitrogen fertilizers enriched with 15N.

Site Latitude Longitude Altitude Mean Annual

Precipitation

Mean Annual

Temperature Physiographic Region Soil Taxonomic Class Soil Series

Soil

Drainage

Class

(m) (cm) (°C)

VA 37.440087 78.662396 197 109 13 Southern Piedmont fine, mixed, subactive, mesic

Typic Hapludults Littlejoe Well

SC 33.869400 79.289300 2 125 17 Atlantic Coast Flatwoods fine, kaolinitic, thermic

Umbric Paleaquults Byars Poorly

NC 35.317006 78.514167 0.5 121 16 Atlantic Coast Flatwoods fine-silty, mixed, active, thermic

Typic Albaquults Leaf Poorly

AR 33.422310 91.732651 69 140 16 Western Gulf Coastal

Plain fine-silty, mixed, active, thermic,

Typic Glossaqualfs Calhoun Poorly

ALN 33.233371 87.232384 158 125 17 Appalachian Plateau

loamy-skeletal, mixed, subactive, thermic, shallow,

Typic Dystrudepts

Montevallo Well

ALS 31.659464 86.272045 111 145 26 Southern Coastal Plain fine, smectitic, thermic,

Typic Hapludults Arundel Well

J.E. Raymond

122

Table 3.2. Selected characteristics of loblolly pine stands in the southern United States selected

to evaluate selected to evaluate temporal foliar uptake of fertilizer nitrogen following the

application of urea or enhanced efficiency nitrogen fertilizers enriched with 15N.

Site Age Trees/Hectare Height (m) DBH (cm) BA (m2 ha-1)

VA 17 1100-1200 13.1 16.5 26.8

SC 16 500-600 16.0 25.7 29.7

NC 18 100-200 18.7 27.2 7.7

AR 13 300-400 15.3 20.8 22.4

ALN 19 300-400 18.8 24.2 25.2

ALS 19 500-600 18.5 21.1 20.4

1

J.E. Raymond

123

Table 3.3. The number of study sites where flushes were observed for each treatment (control, CUF, NBPT, PCU, urea) in sampling

periods after a spring and summer fertilization for loblolly pine stands in the southern United States selected to evaluate N uptake of

urea or enhanced efficiency N containing fertilizers enriched with 15N. (*) represents date of fertilization. (-) represents potential

flushes that were not measured in the sampling period.

Spring Fertilization

Calendar Date 3/27-4/8* 5/8-5/13 6/20-6/30 7/31-8/4 9/11-9/15 10/25-10/27

Weeks after Fertilization 0 6 12 18 24 30

Number Flushes Present 0 1 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3

Treatment

Control 6 6 6 6 0 0 6 6 1 0 6 6 6 0 6 6 6 1 6 6 6 1 CUF 6 6 6 6 2 0 6 6 6 0 6 6 6 2 6 6 6 4 6 6 6 6 NBPT 6 6 6 6 2 0 6 6 6 0 6 6 6 2 6 6 6 4 6 6 6 6 PCU 6 6 6 6 2 0 6 6 6 0 6 6 6 2 6 6 6 4 6 6 6 6 Urea 6 6 6 6 2 0 6 6 6 0 6 6 6 2 6 6 6 4 6 6 6 6

Summer Fertilization Calendar Date 3/27-4/8* 5/8-5/13 6/20-6/30* 7/31-8/4 9/11-9/15 10/25-10/27

Weeks after Fertilization - - 0 6 12 18

Number Flushes Present 0 1 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3

Treatment

Control - - - - - - 6 6 1 0 6 6 6 0 6 6 6 0 6 6 6 1 CUF - - - - - - 6 6 2 0 6 6 6 0 6 6 6 2 6 6 6 3 NBPT - - - - - - 6 6 1 0 6 6 6 0 6 6 6 1 6 6 6 3 PCU - - - - - - 6 6 2 0 6 6 6 0 6 6 6 1 6 6 6 3 Urea - - - - - - 6 6 1 0 6 6 6 0 6 6 6 3 6 6 6 3

1

J.E. Raymond

124

Table 3.4. The mean individual fascicle mass (g) of flushes in sampling periods after a spring and summer fertilization for loblolly

pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers

enriched with 15N. Different letters represent significant differences at α = 0.05 among treatments in a sampling period. Numbers in

parentheses are the standard error of the mean, and absent values and letters indicate post-hoc analysis not conducted due to a

limited sample size. The N application rate was 224 kg N ha-1. N = 6.

Spring Fertilization Date 5/8-5/13 6/20-6/30 7/31-8/4 9/11-9/15 10/25-10/27 Weeks after

fertilization 6 12 18 24 30

Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control 0.16a (0.02)

0.15a (0.04)

- 0.14a (0.02)

0.19a (0.02)

0.07 0.19a (0.02)

0.17a (0.01)

0.17a (0.01)

0.21a (0.0)

0.18a

(0.02) 0.15a (0.02)

0.19 0.19a (0.02)

0.17a (0.01)

0.16a (0.01)

0.19

CUF 0.18a (0.02)

0.18a (0.03)

0.04 0.14a (0.01)

0.21a (0.01)

0.08 0.21a (0.01)

0.24b

(0.01)

0.22b

(0.01)

0.21a (0.02)

0.25b

(0.02)

0.21b

(0.02) 0.24

0.21a (0.02)

0.23b

(0.01)

0.25c

(0.01) 0.24

NBPT 0.17a (0.02)

0.18a (0.03)

0.02 0.15a (0.02)

0.19a (0.02)

0.08 0.19a (0.02)

0.24b

(0.01)

0.21ab (0.03)

0.18a (0.02)

0.22ab (0.01)

0.21b

(0.02) 0.26

0.20a (0.01)

0.25b

(0.01)

0.22bc

(0.01) 0.26

PCU 0.19a (0.02)

0.16a (0.03)

0.03 0.16a (0.02)

0.17a (0.03)

0.06 0.17a (0.03)

0.24b

(0.02)

0.20ab (0.02)

0.18a (0.01)

0.23b

(0.02)

0.18ab (0.03)

0.25 0.20a (0.02)

0.24b

(0.02)

0.19ab (0.02)

0.25

Urea 0.16a (0.02)

0.19a (0.02)

0.05 0.15a (0.02)

0.20a (0.02)

0.06 0.20a (0.02)

0.24b

(0.01)

0.22b

(0.02)

0.18a (0.02)

0.21ab (0.01)

0.19ab (0.01)

0.23 0.18a (0.02)

0.24b

(0.02)

0.23bc

(0.02) 0.23

Summer Fertilization

Date 5/8-5/13 6/20-6/30 7/31-8/4 9/11-9/15 10/25-10/27 Weeks after

fertilization - - 6 12 18

Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control - - - - - - 0.20a (0.01)

0.20a (0.01)

0.16a (0.01)

0.18a (0.01)

0.18a (0.01)

0.17a (0.01)

- 0.19a (0.02)

0.20a (0.02)

0.18a (0.01)

0.22

CUF - - - - - - 0.21a (0.01)

0.22ab (0.02)

0.22b

(0.02)

0.21a (0.02)

0.21ab (0.02)

0.20ab (0.03)

0.16 (0.01)

0.19a (0.02)

0.24b

(0.03) 0.19ab (0.02)

0.20 (0.02)

NBPT - - - - - - 0.21a (0.01)

0.25c

(0.01)

0.23b

(0.01)

0.23a (0.01)

0.23b

(0.01)

0.23b

(0.01) 0.15

0.19a (0.02)

0.24ab (0.02)

0.24b

(0.01)

0.20 (0.02)

PCU - - - - - - 0.21a (0.01)

0.23bc

(0.01)

0.20b

(0.01)

0.21a (0.03)

0.21ab (0.03)

0.20ab (0.02)

0.17 0.17a (0.03)

0.24b

(0.01) 0.21ab (0.02)

0.20 (0.01)

Urea - - - - - - 0.21a (0.01)

0.24bc

(0.01)

0.20b

(0.01)

0.24a (0.01)

0.24b

(0.01)

0.23b

(0.02)

0.17 (0.01)

0.21a (0.01)

0.26b

(0.01)

0.25b

(0.01) 0.23

(0.04)

J.E. Raymond

125

Table 3.5. The mean foliar N concentration (g kg-1) of individual flushes for sampling periods after a spring and summer fertilization

for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N

containing fertilizers enriched with 15N. Different letters represent significant differences at α = 0.05 among treatments in a sampling

period. Numbers in parentheses are the standard error of the mean, and absent values and letters indicate post-hoc analysis not

conducted due to a limited sample size. The N application rate was 224 kg N ha-1. N = 6.



Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control 10.8a (0.5)

11.2a (0.8)

- 10.8a (0.4)

11.0a (0.4)

10.6 10.3a (0.5)

11.6a (0.8)

11.4a (0.7)

9.6a (0.7)

11.4a (1.0)

10.5a (1.0)

10.2 8.6a (0.6)

10.9a (0.8)

11.7a (0.6)

10.2

CUF 12.8a (0.5)

10.4a (0.7)

8.6 11.9a (0.7)

12.1a (0.4)

12.9 12.7a (0.9)

14.0a (0.8)

12.8ab (0.7)

12.0a (0.9)

13.6a (1.3)

13.1a (1.2)

12.9 11.5ab (0.6)

12.3a (1.3)

14.0a (0.5)

13.7

NBPT 12.1a (0.8)

12.5a (0.8)

6.8 12.5a (1.0)

13.0a (1.3)

13.9 13.0a (1.2)

15.3a (1.3)

16.0b

(1.7)

11.4a (0.6)

13.4a (1.6)

13.8a (1.2)

13.5 13.0b

(1.6)

14.5a (1.6)

14.1a (0.7)

13.8

PCU 11.1a (0.2)

11.7a (0.7)

7.1 11.7a (0.5)

12.4a (0.7)

10.1 12.3a (0.5)

14.4a (0.8)

12.8ab (0.6)

11.3a (0.7)

12.8a (1.1)

13.4a (0.7)

14.1 11.5ab (0.5)

12.4a (1.0)

13.9a (0.4)

14.1

Urea 12.5a (0.4)

12.8a (1.2)

7.7 12.2a (0.9)

12.8a (1.0)

12.0 12.2a (0.7)

14.4a (1.4)

13.8ab (1.6)

12.4a (0.5)

13.7a (1.2)

13.3a (0.9)

12.8 12.6b

(0.7)

12.3a (1.0)

13.9a (0.8)

12.8

Summer Fertilization Date 5/8-5/13 6/20-6/30 7/31-8/4 9/11-9/15 10/25-10/27 Weeks after


Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control - - - - - - 9.8a (0.5)

11.5a (0.9)

11.1a (0.9)

10.0a (0.8)

10.2a (0.6)

10.4a (0.7)

- 10.3a (1.0)

10.9a (1.0)

11.8a (0.4)

10.5

CUF - - - - - - 11.6a (0.7)

14.5a (1.4)

13.6a (1.4)

10.6a (0.7)

13.6a (1.6)

13.1ab (0.6)

14.4 10.8a (0.9)

12.9a

(1.4) 13.3a (0.8)

13.6

NBPT - - - - - - 12.7a (1.0)

14.1a (0.9)

14.2a (1.1)

12.8a (1.3)

13.2a (1.1)

14.1ab (0.9)

14.6 11.9a (0.8)

12.5a (1.2)

13.8a (0.9)

15.4

PCU - - - - - - 11.9a (0.7)

12.9a (1.0)

11.6a (1.2)

1.13a (0.6)

13.2a (1.0)

13.3ab (1.2)

16.3 10.0a (1.2)

11.5a (0.8)

12.8a (1.0)

12.7

Urea - - - - - - 12.6a (0.6)

14.1a (0.6)

14.1a (1.0)

11.6a (1.1)

14.0a (1.2)

14.2b

(1.0) 14.8

11.3a (0.7)

13.4a (0.9)

13.9a (0.9)

15.4

J.E. Raymond

126

Table 3.6. The mean individual fascicle N content (mg N fascicle-1) of flushes per sampling periods after a spring and summer

fertilization for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced

efficiency N containing fertilizers enriched with 15N. Different letters represent significant differences at α = 0.05 among treatments in

a sampling period. Numbers in parentheses are the standard error of the mean, and absent values and letters indicate post-hoc

analysis not conducted due to a limited sample size. The N application rate was 224 kg N ha-1. N = 6.



Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control 1.69a (0.28)

1.65a (0.49)

- 1.95a (0.24)

1.49a (0.23)

0.67 2.02a (0.21)

1.93a (0.21)

1.82a (0.16)

2.03a (0.23)

2.02a (0.32)

1.59a (0.34)

1.90 1.74a (0.03)

1.91a (0.02)

1.91a (0.01)

1.91

CUF 2.24a (0.28)

1.87a (0.28)

0.56 1.64a (0.34)

1.69a (0.18)

1.06 (0.04)

2.60a (0.12)

3.35b

(0.36)

2.84b

(0.13)

2.53a (0.36)

3.45b

(0.54)

2.75b

(0.40)

3.15 (0.79)

2.41ab (0.02)

2.72b

(0.02)

3.51c

(0.03) 3.62

NBPT 2.04a (0.26)

2.38a (0.50)

0.58 1.82a (0.39)

2.13a (0.34)

1.12 (0.06)

2.47a (0.25)

3.57b

(0.18)

3.29b

(0.50)

2.11a (0.22)

3.01b

(0.45)

3.00b

(0.43)

3.49 (0.72)

2.72b

(0.04)

3.62c

(0.05)

3.22bc

(0.03) 2.92

PCU 2.13a (0.24)

1.83a (0.41)

0.21 1.69a (0.29)

1.95a (0.20)

0.62 (.20)

2.04a (0.29)

3.43b

(0.29)

2.57ab (0.32)

2.08a (0.26)

2.93b

(0.37)

2.43b

(0.38)

3.56 (1.03)

2.21ab (0.03)

2.91bc

(0.02)

2.75b

(0.02) 3.61

Urea 2.04a (0.04)

2.36a (0.36)

0.48 1.43a (0.28)

1.92a (0.27)

0.76 (0.15)

2.42a (0.03)

3.44b

(0.41)

3.08b

(0.53)

2.18a (0.23)

2.86b

(0.40)

2.54b

(0.39)

2.92 (0.47)

2.31ab (0.03)

2.34bc

(0.03)

3.21bc

(0.04) 2.91


fertilization 6 12 18

Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control - - - - - - 1.99a (0.20)

2.25a (0.02)

1.78a (0.22)

1.98a (0.24)

1.77a (0.18)

1.18a (0.02) -

1.95a (0.30)

2.15a (0.28)

2.11a (0.15)

2.31

CUF - - - - - - 2.45ab (0.24)

3.21b

(0.49)

2.98c

(0.41)

1.85a (0..23)

2.68b

(0.36)

2.71b

(0.04)

2.41 (0.37)

2.10a (0.39)

3.19b

(0.51)

2.46b

(0.26)

2.82 (0.47)

NBPT - - - - - - 2.58b

(0.16)

3.45b

(0.27)

3.11c

(0.30)

2.64b

(0.43)

3.18b

(0.21)

3.23b

(0.02) 2.24

2.26a (0.35)

2.94ab (0.37)

3.29c

(0.28)

3.12 (0.32)

PCU - - - - - - 2.53ab (0.20)

2.94b

(0.82)

2.39b

(0.36)

2.19ab (0.20)

2.69b

(0.39)

2.72b

(0.04) 2.81

1.86a (0.33)

2.91ab (0.16)

2.78b

(0.33)

2.64 (0.61)

Urea - - - - - - 2.57ab (0.10)

3.38b

(0.35)

2.86bc

(0.21)

2.27ab (0.17)

3.26b

(0.33)

3.31b

(0.03)

2.64 (0.18)

2.43a (0.28)

3.46b

(0.24)

3.46c

(0.32)

3.60 (0.62)

J.E. Raymond

127

Table 3.7. The mean foliar 15N (‰) of individual flushes for sampling periods after a spring and summer fertilization for loblolly pine

stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N containing fertilizers

enriched with 15N. Different letters represent significant differences at α = 0.05 among treatments in a sampling period. Numbers in

parentheses are the standard error of the mean, and absent values and letters indicate post-hoc analysis not conducted due to a

limited sample size. The N application rate was 224 kg N ha-1. N = 6.



Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control -3.22a (1.0)

-2.61a (1.2)

- -2.96a (0.9)

-3.07a (1.1)

-2.80

-3.43a (0.6)

-3.28a (1.0)

-1.56a (0.7)

-3.38a (1.0)

-3.94a (1.3)

-2.84a (0.9)

0.41 -2.50a (0.73)

-3.15a (0.89)

-2.92a (0.58)

0.42

CUF 36.3a (4.1)

70.0bc

(11.0) 45.6 58.8b

(7.2)

92.6c

(13.9)

106.7

69.9b

(8.3)

114.4b

(18.1)

123.1b

(10.2)

56.7b

(8.5)

92.1b

(10.6)

113.4b

(10.7) 111.2 65.4b

(7.4)

95.4b

(12.2)

120.1b

(11.2) 114.4

NBPT 32.0a (3.0)

89.4c

(19.5) 38.5

43.9ab (7.5)

90.2bc

(16.6) 87.6 57.7b

(7.8)

100.8b

(17.5)

109.1b

(15.0)

56.3b

(6.0)

84.0b

(8.0)

116.3b

(12.2) 92.8 64.6b

(10.5)

94.0b

(14.3)

105.5b

(14.5) 134.2

PCU 5.37a (2.9)

17.8ab (7.03)

9.72

24.5ab (7.5)

40.9ab (8.7)

60.4 44.0ab (7.4)

76.8b

(12.5)

85.4b

(12.3)

48.4b (6.3)

77.2b

(13.9)

80.3b

(5.7) 106.1 53.5b

(9.3)

73.8b

(7.4)

107.2b

(11.5) 106.1

Urea 31.1a (2.5)

58.6bc

(11.2) 41.2

47.4ab (7.5)

83.2bc

(13.7)

106.4

60.4b

(6.9)

96.3b

(17.3)

103.5b

(14.2)

65.9b

(18.6)

90.7b

(12.3)

112.1b

(7.4) 119.3 80.1b

(11.8)

94.4b

(12.5)

117.9b

(8.1) 119.3



Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control - - - - - - -3.11a (0.70)

-2.90a (0.66)

-2.10a (0.93)

-3.04a (1.01)

-2.84a (0.81)

-2.55a (0.89)

- -2.87a (0.66)

-3.14a (0.81)

-2.84a (0.81)

0.51

CUF - - - - - - 35.2a (8.3)

61.7b

(13.3) 72.8c

(19.1)

49.5b

(6.3)

73.1b

(8.9)

76.0b

(10.3) 92.1 58.7b

(9.4)

69.4b

(9.9)

102.6b

(16.4) 109.2

NBPT - - - - - - 33.6a (6.2)

52.1b

(8.6) 64.8bc

(10.4)

53.3b

(5.4)

75.0b

(9.2)

99.3b

(11.3) 158.9 60.9b

(8.0)

65.5b

(9.7)

90.5b

(11.5) 118.6

PCU - - - - - - 21.0a (7.0)

22.9ab (6.2)

28.6ab (5.9)

40.5b

(3.8)

52.4b

(5.9)

67.2b

(10.9) 58.7 40.8b

(5.8)

45.6b

(5.2)

58.0b

(7.8) 76.99

Urea - - - - - - 30.3a (7.7)

59.2b

(5.9) 66.5bc

(9.6)

47.1b

(6.4)

63.7b

(9.9)

82.6b

(7.6) 95.0 47.7b

(10.0)

68.4b

(7.1)

86.3b

(9.01) 94.64

1

2

J.E. Raymond

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Table 3.8. The mean individual fascicle fertilizer N (mg N fascicle-1) content for flushes in sampling periods of loblolly pine stands in

the southern United States selected to evaluate seasonal fertilizer N uptake of urea or enhanced efficiency N containing fertilizers

enriched with 15N after a spring and summer fertilization. Different letters represent significant differences at α = 0.05 among

treatments in a sampling period. Numbers in parentheses are the standard error of the mean, and absent values and letters indicate

post-hoc analysis not conducted due to a limited sample size. The N application rate was 224 kg N ha-1. N = 6.



Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control 0.00a 0.00a 0.00 0.00a 0.00a 0.00 0.00a 0.00a 0.00a 0.00a 0.00a 0.00a 0.00 0.00a 0.00a 0.00a 0.00

CUF 0.19a (0.03)

0.29b

(0.05) 0.14 0.25c

(0.08)

0.34bc

(0.04) 0.28 0.43b

(0.06)

0.88b

(0.13)

0.82b

(0.07)

0.31b

(0.04)

0.76b

(0.16)

0.72bc

(0.10) 0.73 0.35b

(0.02)

0.62bc

(0.11)

0.97b

(0.09) 0.84

NBPT 0.15a (0.02)

0.57c

(0.21) 0.16

0.20bc

(0.07)

0.43c

(0.08) 0.24

0.36b

(0.06)

0.90b

(0.18)

0.74b

(0.22)

0.29b

(0.04)

0.64b

(0.13)

0.87c

(0.20) 0.50

0.45b

(0.14)

0.84c

(0.16)

0.82b

(0.15) 0.91

PCU 0.03a (0.01)

0.07a (0.02)

0.03 0.11ab (0.04)

0.20b

(0.05) 0.09

0.25ab (0.06)

0.68b

(0.14)

0.54b

(0.09)

0.25ab (0.03)

0.60b

(0.14)

0.49b

(0.08) 0.86 0.30b

(0.06)

0.54b

(0.06)

0.71b

(0.09) 0.86

Urea 0.16a (0.08)

0.33b

(0.08) 0.09

0.18bc

(0.05)

0.39c

(0.09) 0.14

0.37b

(0.10)

0.84b

(0.21)

0.66b

(0.21)

0.33ab

(0.09)

0.66b

(0.17)

0.68bc

(0.09) 0.87

0.45b

(0.11)

0.73bc

(0.17)

0.93b

(0.16) 0.87

Summer Fertilization

Date 5/8-5/13 6/20-6/30 7/31-8/4 9/11-9/15 10/25-10/27 Weeks after

fertilization 6 12 18

Flush 0 1 2 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Control - - - - - - 0.00a 0.00a 0.00a 0.00a 0.00a 0.00a 0.00 0.00a 0.00a 0.00a 0.00

CUF - - - - - - 0.20b

(0.05)

0.48b

(0.13)

0.50b

(0.13)

0.21b

(0.03) 0.50b

(0.09)

0.45b

(0.05) 0.19 0.32b

(0.08)

0.54c

(0.11)

0.58c

(0.10) 0.58

NBPT - - - - - - 0.22b

(0.04)

0.44b

(0.09) 0.49b

(0.09) 0.35b

(0.07) 0.60b

(0.12) 0.78c

(0.12) 0.23 0.35b

(0.09)

0.50bc

(0.12)

0.74c

(0.14) 0.91

PCU - - - - - - 0.14ab (0.05)

0.16a (0.04)

0.16a (0.03)

0.22b

(0.03) 0.37b

(0.07) 0.43b

(0.07) 0.18 0.19ab

(0.07) 0.33b

(0.05) 0.40b

(0.07) 0.59

Urea - - - - - - 0.18b

(0.05)

0.47b

(0.05)

0.44b

(0.06) 0.26b

(0.05) 0.50b

(0.08) 0.65c

(0.09) 0.29 0.65c

(0.09)

0.57c

(0.07) 0.70c

(0.08) 0.48

1

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Chapter 4. Understanding the fate of applied fertilizer nitrogen in mid-rotation

loblolly pine plantations (Pinus taeda L) of the southern United States using stable isotopes

Abstract

The recovery of applied fertilizer nitrogen (N) following surface application of urea and

three enhanced efficiency N containing fertilizers (EEFs) were compared in two studies (Study 1,

Study 2) at eighteen thinned mid-rotation loblolly pine stands (Pinus taeda L.) across the

southern United States. All fertilizer treatments were labeled with 15N (~370 permil, 0.5 AP) and

applied during two different seasons (spring, summer) in Study 1 in 2011, and a single season

(spring) in Study 2 in 2012 to 100 m2 circular plots. After N fertilization, all ecosystem

components were sampled at the end of a single growing season to determine the ecosystem

partitioning of fertilizer N. Total fertilizer N recovery for each treatment was determined using a

mass balance calculation for individual ecosystem components. Significantly more fertilizer N

was recovered for all EEFs compared to urea for both Study 1 and Study 2, although there were

no differences among EEFs. There were no differences in fertilizer N recovery between the

season of application for Study 1. For Study 1, total fertilizer N recovery ranged from 78.8% to

82.1% for EEFs compared to 64.6% for urea, and for Study 2 total fertilizer N recovery ranged

from 77.1% to 79.5% for EEFs compared to 55.5% for urea. Most of the fertilizer N recovered

for all treatments was in the loblolly pines or soil. Recovery in loblolly pine for urea was 34.8%

for Study 1 and 33.8% for Study 2. For the EEFs, fertilizer N recovery in loblolly pines for Study

1 ranged from 38.5% to 49.9%, and in Study 2 ranged from 33.2% to 42.1%. There was an

increasing amount of fertilizer N recovered with EEFs in the soil for both studies (Study 1 =

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30.5% to 38.7%, Study 2 = 35.5% to 42.5%) compared to urea (Study 1 = 28.4%, Study 2 =

20.5%). This research highlights increased fertilizer N recovery and ecosystem partitioning of

fertilizer N for EEFs compared to urea in southern loblolly pine plantations, potentially

increasing fertilizer N use efficiency in these pine plantations.

131

4.1. Introduction

Loblolly pine (Pinus taeda L.) is the most widely planted tree species in the United

States (USDA 2012), extending from southeast Oklahoma and east Texas to north Florida

and southern New Jersey (Shultz 1997). Loblolly pine plantations are often intensively

managed through the planting of improved genetic stock, site preparation, competition

control, and fertilization (Fox et al. 2007a). The combined interactions of these treatments

often doubles loblolly pine growth, with many stands producing more than 10 m3 ha-1 yr-1

(Fox et al. 2007a). One primary constraint limiting growth in many loblolly pine plantations

in the South is the deficiency of plant nutrients, specifically nitrogen (N) and phosphorous (P)

(Allen 1987, Fox et al. 2007b). Soil N deficiencies are common in the South because larger

quantities of N are required by trees compared to other nutrients, and most forest soils are

unable to adequately supply plant available N to meet tree demand (Miller 1981, Chapin et al.

1986, Vitousek and Howarth 1991). Low N availability restricts leaf area production which

decreases photosynthetic capacity, net primary productivity and hence the growth of the stand

(Linder 1987, LeBauer and Treseder 2008).

Tree N uptake requirements increase with stand age (Switzer and Nelson 1972, Wells

and Jorgenson 1979, Fox et al. 2007a) and leads to nutrient limitations in loblolly pine

plantations when the potential nutrient use of the stand is not provided by the soil nutrient supply

(Allen et al. 1990). The ecosystem disturbance associated with stand establishment activities

increases soil N availability due to N mineralization from the soil organic matter and plant

residues produced by harvesting activities (Vitousek and Matson 1985, Fox et al. 1986). As a

stand ages, plant available N declines because of increased N immobilization within the

ecosystem (Birk and Vitousek 1986). To ameliorate reduced plant N availability, fertilization

with N is required to maintain forest productivity (Miller 1981). On average, southern loblolly

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pine plantation growth increases 3 m3 ha-1 yr-1 up to 8 years following a mid-rotation

application of 224 kg ha-1 N plus 28 kg ha-1 of P (Fox et al. 2007a). These results have led to

the annual fertilization of over 500,000 ha of pine plantations in the southern United States

(Albaugh et al. 2007). However, certain sites do not respond, or respond negatively, to

fertilization (Pritchett and Smith 1975, Martin et al. 1999, Amateis et al. 2000, Carlson et al.

2014). These inconsistent fertilizer trial results have led to a wide range of productivity in pine

plantations (Rojas 2005, Fox et al. 2007a).

Despite fertilization improving the growth of most loblolly pine plantations, tree N

uptake is generally less than 25% of applied fertilizer N (Baker et al. 1974, Mead and

Pritchett 1975, Melin et al. 1983, Ballard 1984, Johnson and Todd 1988, Johnson 1992, Li et

al. 1999, Albaugh et al. 2004, Blazier et al. 2006), although some studies have reported larger

amounts of N uptake (Raison et al. 1990). Explanations for low fertilizer N uptake in field

studies include: 1) fertilizer N loss from the system following fertilizer application; 2) variable

fertilizer N immobilization in ecosystem components with fluctuating turnover rates; and 3)

asynchronous soil supply of fertilizer N with seasonal plant N demand, all which reduce

fertilizer N use efficiency (FNUE). As with all agroecosystems, an improved understanding of

fertilizer N uptake and ecosystem retention in southern pine plantations is required to enhance

FNUE to continue to increase growth and productivity of these systems in the future (Zhang et

al. 2015).

To improve FNUE in agroecosystems, slow release (SRN), controlled release (CRN)

and stabilized (SNF) N fertilizers were developed (Azeem et al. 2014, Fertilizer Institute

2015). CRN is fertilizer N that is coated or encapsulated to alter the rate, pattern and duration

of N release (Chien et al. 2009). SRN are N fertilizer formulations that gradually release

fertilizer N slowly as a result of microbial decomposition (Trenkel 1997). SNF is fertilizer N

that is treated with chemical compounds that inhibit rapid N transformation to less stable

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forms (Shaviv 1996, 2005). The SRN, CRN and SNF products can all be broadly categorized

as enhanced efficiency fertilizers (EEFs) (AAPFCO 2015). The Fertilizer Institute (2015)

defines EEFs as fertilizer products that reduce nutrient loss to the environment, increase plant

nutrient availability by slowing nutrient release, and the conversion of the nutrient to forms

less susceptible to losses. The N containing EEFs are designed to: 1) increase N uptake by the

crop; 2) decrease N leaching; 3) lower toxicity; 4) provide extended N supply; 5) reduce

volatilization and/or nitrification losses of N; and 6) lower application cost (Allen 1984,

Hauck 1985). Therefore, EEFs can reduce fertilizer N loss and provide greater flexibility for

fertilization under a variety of weather conditions and enhance fertilizer N uptake by the crop

(Hauck 1985, Goertz 1993, Azeem et al. 2014). Although EEFs were developed to address

FNUE issues in agroecosystems, similar issues related to FNUE exist for southern pine

plantations. Implementing EEF technology into forest fertilization programs in southern pine

plantations may improve FNUE and increase the productivity of these systems.

Pelletized urea (46-0-0) is the primary N fertilizer used in southern pine plantations

because of its high N content and lowest overall cost per unit of applied N (Allen 1987, Fox

et al. 2007b). Several chemical reactions occur after urea is applied in the environment

(Hauck and Stephenson 1965, Ferguson et al. 1988). The initial reaction, urea hydrolysis, is

catalyzed by the extracellular enzyme urease which produces ammonium bicarbonate

(Conrad 1942, Pettit et al. 1976). Ammonium bicarbonate dissociates into ammonium (NH4+)

and bicarbonate (HCO3-). The bicarbonate consumes hydrogen (H+), raising the microsite pH

surrounding the urea granule. The elevated microsite pH surrounding the urea granules cause

ammonium ions (NH4+) to dissociate to ammonia (NH3) that can be volatilized to the

atmosphere. High losses of fertilizer N after urea fertilization can occur through NH3

volatilization and depend on the interaction of weather conditions and edaphic site factors at the

time of fertilization (Cabrera et al. 2005, Zerpa and Fox 2011, Elliot and Fox 2014, Raymond et

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al. 2016). Substantial fertilizer N loss after urea fertilization in southern loblolly pine plantations

translates to less fertilizer N remaining in the ecosystem, and may lead to less fertilizer N

availability for tree N uptake. To reduce NH3 volatilization, urea application in southern pine

plantations usually occurs in the winter when temperatures are lower and precipitation is high.

However, significant N loss through NH3 volatilization can still occur during the winter (Engel

et al. 2011, Elliot and Fox 2014). Other N loss pathways may still be significant at this time

including denitrification (Shrestha et al. 2014) or leaching (Binkely et al. 1995, Aust and

Blinn 2004). Fertilizer N lost immediately following fertilization translates to less fertilizer N

remaining in the ecosystem and a lower FNUE (Galloway et al. 2003).

One SNF mechanism to reduce urea N loss is to add urease inhibitors to urea prior to

fertilization. Urease inhibitors are substances which temporarily inhibit the hydrolytic action

on urea by the urease enzyme, resulting in less urea N lost through NH3 volatilization

(AAPFCO 2015). These products generally reduce urease activity for two to three weeks

after fertilizer application (Gowariker and Krishnamurty 2009, Havlin et al. 2014), although

under certain conditions effectiveness can be longer (Engel et al. 2011). Urease is abundant in

soils and produced by numerous soil organisms and plants (Bremner and Douglas 1971,

Bremner and Chai 1986, Antisari et al. 1996, Sanz-Cobena et al. 2008). When urease

inhibitors are released to the environment, they bind to the urease enzyme which reduces the

activity of the enzyme and provides time for urea to dissolve and move into the soil. Once in

the soil, loss via NH3 volatilization from urea is reduced because the soil buffers the pH.

Many compounds inhibit urease activity, but few are chemically stable when added to urea,

effective when applied at low concentrations, and are non-toxic when applied in the

environment (Hauck 1985, Azeem et al. 2014). The most widely used urease inhibitor is N-

(n-Butyl) triphosphoric triamide (NBPT) (Havlin et al. 2014). Urea impregnated with NBPT

can reduce losses of fertilizer N from NH3 volatilization significantly compared to untreated

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urea (Engel et al. 2011, Raymond et al. 2016) and may translate to improved productivity for

desired species (Engel et al. 2011).

The CRN products generally have a urea granule base which is initially coated with a

compound (i.e. sulfur (S), boron, (B), copper (Cu)) and additionally sealed with a wax-like

substance to fill imperfections (Chien et al. 2009). Fertilizer N release from CRN depends on

the thickness and coating quality (Allen 1984). As the coating degrades in the environment,

cracks develop in the coating allowing moisture to enter the product, and urea dissolution is

initiated. Urea N release to the environment is slowed by the coating compared to uncoated

urea. The substrate coating of CRN may reduce urease activity, and CRNs may also be

impregnated with urease or nitrification inhibitors to slow loss mechanisms as urea N is

released to the environment. Additionally, CRN products may include other nutrients in the

coatings to address additional nutrient deficiencies.

An alternative CRN approach encapsulates fertilizer N (generally urea) in a semi-

permeable polymer coating. The coatings have small pores which allow water to enter the

capsules. As water enters the CRN product, urea dissolution occurs, the capsule expands and

the resulting internal pressure of the capsule forces fertilizer N solution into the environment

through diffusion (Shaviv 2005). Because of the low mass of the polymer coatings, these

products do not significantly reduce the N percentage in the product as may occur with the

aforementioned CRN products (Chien et al. 2009). Release patters for polymer coated CRN

products are related to the type of coating, soil moisture, temperature, relative humidity,

precipitation, pH, and microbial activity (Christianson 1988, Shoji et. al. 2001, Shaviv 2005).

Although tree growth is often N limited, forest soils contain large quantities of N,

most of which is unavailable for plant uptake (Fisher and Binkley 2000). Because of the large

amount of N present in forest soils, ranging from 2 to 7 Mg ha-1, it is difficult to distinguish

changes in soil N following a single or multiple applications of fertilizer N, which typically

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adds only 150 kg ha-1 to 250 kg ha-1 of N (Fisher and Binkley 2000, Flint 2007, Kiser and Fox

2014). However, fertilizers enriched with the stable isotope 15N can be used to trace the fate

of fertilizer N in the ecosystem to improve our understanding of mechanisms and cycling of

applied N (Knowles and Blackburn 1993, Nadelhoffer and Fry 1994, Robinson 2001,

Dawson et al. 2002).

Numerous 15N tracer studies have been conducted in agroecosystems to improve our

understanding of fertilizer N cycling in crop systems (Hauck and Bystrom 1971). Although

research using 15N to trace the fate of fertilizer N has been conducted in forest, most recent

work with 15N in forest ecosystems has focused on the natural cycling of N (Currie and

Nadelhoffer 1999, Dinkelmeyer et al. 2003, Nadelhoffer et al. 2004) and/or the potential

negative effects of increased N deposition from industrialization (Tietema et al. 1998,

Templar et al. 2012) in natural systems. Research using 15N enriched fertilizers to study

FNUE in intensively managed forest plantations have been surprisingly limited (Hauck 1968,

Mead and Pritchett 1975, Melin et al. 1983, Chang et al. 1996, Bubb et al. 1999, Werner

2014).

Using 15N enriched fertilizers to study fertilizer N uptake and ecosystem fate in

southern pine plantations will improve our fundamental understanding of the fate of fertilizer

N in these systems. Combined with enhanced efficiency fertilizer technology, this improved

understanding will increase FNUE efficiency in southern pine plantations into the future. The

statistical hypotheses for this research are:

HO1: There are no differences in the total fertilizer N recovery between urea and the

enhanced efficiency N containing fertilizers.

HO2: There are no differences in total fertilizer N recovery between urea and the enhanced

efficiency N containing fertilizers between a spring and summer fertilizer application.

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HO3: There are no differences in the total fertilizer N recovery between urea and the

enhanced efficiency N containing fertilizers for each primary ecosystem component.

4.2. Methods

4.2.1. Experimental Design and Site Description

Two separate experiments (Study 1, Study 2) were designed to test differences of total

fertilizer N recovery among fertilizer treatments using urea and three EEFs in southern

loblolly pine plantations. Study 1 (2011) used a split plot complete block design to test

differences between five fertilizer treatments (main plots) at two different application seasons

(split plot). Five treatments (CUF, NBPT, PCU, urea, control) were applied at six sites during

2011 in two separate seasons (spring: March-April; summer: June) in mid-rotation loblolly

pine plantations across the southern United States (Figure 4.1). The five fertilizer treatments

were the main plots with single replication at each individual site. Each individual site served

as a block. The split plot was the application season (spring vs. summer). Study 2 (2012)

expanded the geographic scope of Study 1 and evaluated the same five treatments at 12

additional sites in one application season (spring) (Figure 4.1). Study 2 was a complete block

design where fertilizer treatments served as the treatment plots with single replication at each

individual site, and each individual site served as a block. Selected physical site

characteristics of the sites used in Study 1 and Study 2 are described in Tables 4.1 and 4.2,

and selected soil chemical and physical data is detailed in Table 4.3.


The five treatments were used in both Study 1 and Study 2. The five treatments were:

1) urea; 2) urea impregnated with N-(n-Butyl) thiophosphoric triamide (NBPT); 3) urea

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impregnated with NBPT and coated with monoammonium phosphate (CUF); 4) urea coated

with a polymer (PCU); and 5) a control treatment where no fertilizer N was added. Urea (46-

0-0) was the conventional N fertilizer used because of its common use in southern forests.

The enhanced efficiency fertilizers (EEFs) tested in this study were developed to reduce NH3

volatilization and release fertilizer N slowly to the environment. In the NBPT treatment, urea

(46-0-0) granules were impregnated with N-(n-butyl) thiophosphoric triamide at a rate of

26.7% by weight to inhibit urease activity. In the CUF treatment (39-9-0), urea granules were

impregnated with NBPT and coated with an aqueous binder containing a boron and copper

sulfate solution to slow the release of N to the environment. A coating of monoammonium

phosphate was then added to provide P. The PCU (44-0-0) treatment had a polymer coating

covering urea granules with pore openings designed to slowly release N (~80%) over a 120

day period. The application rate for all treatments was equivalent to 224 kg N ha-1. Because

the CUF treatment contained P in the monoammonium phosphate coating, P was added to the

other fertilizer treatment at the equivalent rate of 28 kg P ha-1 using triple superphosphate

(TSP). The urea for all treatments was enriched with the stable isotope 15N (~370‰, 0.5 AP)

(Nadelhoffer and Frye 1994). All individual fertilizer treatments for both studies were hand

broadcast applied to individual 100m2 circular plots described below.

4.2.3. Experimental Method- Field Sampling – Pre-treatment

Five 100 m2 circular plots were installed at each site for each application season for

both studies in areas with similar stand, soils and landscape characteristics and a minimum

distance of 30.5 m from individual plots (Figure 4.2). The plot center was located equidistant

between two co-dominant loblolly pine trees with similar height (HT) and diameter (DBH).

After plot installation and prior to treatment application, the HT and DBH was measured for

all trees greater than 2.54 cm DBH including coniferous and deciduous species. Percent cover

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(PC) was ocular estimated for each of the following understory strata: 1) sapling; 2) shrub; 3)

vine; and 4) herbaceous. Sapling and shrub strata were identified by species. Foliage, fine (<

2 cm) and coarse (> 2 cm) branches were sampled from the two central crop trees from the

upper 1/3rd of the canopy. Foliage (if present) and branches were sampled from individual

species in the sapling and shrub strata. Vines and herbaceous plants were sampled by

composite for respective strata. The forest floor, or O horizon (Oi + Oe + Oa), was sampled

using a 0.07 m2 circular frame at four randomly selected areas and composited. The mineral

soil was sampled with a push-tube auger at 8 random locations for two depths (0-15 cm, 15-

30 cm) and composited by depth. Root samples of the mineral soil was composited from four

random locations with a bulk density sample corer (AMS Inc.) up to a 30 cm depth. Two

circular litterfall traps were randomly placed in each plot to sample litterfall after N

fertlization. Bulk density samples of the 0-15 cm and 15-30 cm depths of mineral soil was

determined for each individual plot using a bulk density sample corer (AMS Inc.).

4.2.4. Experimental Method- Field Sampling – Post-treatment

For both studies, the individual fertilizer N treatments were randomly applied to the

individual 100m2 circular plots on a single day for each application season at each individual

site. The spring application date for both studies was between March 26 to April 8, and

summer application in Study 1 was from June 18 to June 30. All plots were resampled

between November 1st and March 31st at the end of the growing season in both studies using

the same procedures as the pre-treatment sampling. In addition, litterfall was collected from

traps and composited in each plot. One central crop tree per plot was harvested and separated

into the following components: 1) foliage; 2) fine branches; 3) coarse branches; 4) dead

branches; and 5) stem. Each biomass component was weighed in the field to obtain green

weights. Subsamples of each biomass component were returned to the lab and dried to

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determine moisture content which was used to convert biomass to a dry weight basis. Stem

samples (cookies) were cut at DBH, the base of the live crown (HLC), and at mid-crown

(HMC). The sapling stratum, if present, was divided in 2.54 cm individual species diameter

classes, and one tree from each diameter class was harvested and weighed in the field for

green weights. Each plot was divided in 32 equal wedge sections, and one section was

randomly selected to sample the entire shrub, vine and herbaceous strata. Shrubs were

divided by species whereas vine and herbaceous strata were composited in their respective

strata. Samples from each component were put in separate labeled paper lawn and grocery

bags and returned to the laboratory for processing.

4.2.5. Experimental Method - Laboratory Processing

All biomass samples were dried in a forced air oven at 60°C until samples achieved a

constant weight. Litterfall was separated into the subcategories of pine needles, deciduous

leaves, fine branches, coarse branches, bark and unidentifiable litterfall. The stem cookies

collected at DBH were subsampled for bark, wood in the current year of growth when

treatment was applied (CGR), and all wood composited from all the previous growth prior to

fertilizer treatment (PGR). Roots were washed from mineral soil with an elutriation system

and separated into fine (<2 mm) and coarse (>2 mm) and oven dried. The O horizon samples

were sieved through a 6 mm sieve and mineral soil was sieved through a 2 mm sieve. Soil

chemical data presented in Table 4.3 was determined through the Mehlich I procedure at the

Virginia Tech Soil Testing Laboratory.

All individual plant material, litterfall and O horizon samples were initially coarse

ground with a Wiley Mill to pass a 2 mm sieve. Individual samples were then ground to a fine

powder in a ball mill (Retsch® Mixer Mill MM 200, Haan, Germany) with all individual

organic substrate samples ball milled for 1 minute at 25 revolutions per second (rps). Mineral

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soil samples were ball milled for 2 minutes at 25 rps. After ball milling, individual

homogenized samples were put in separate tin capsules and weighed on a Mettler-Toledo©

MX5 microbalance (Mettler-Toledo, Inc., Columbus, OH, USA). Individual samples were

analyzed to determine the 15N/14N isotope ratio, total N and total C on a coupled elemental

analysis-isotope ratio mass spectrometer (IsoPrime 100 EA-IRMS, Isoprime© Ltd.,

Manchester, UK) at the Forest Soils and Plant Nutrition Laboratory at Virginia Polytechnic

Institute and State University (Virginia Tech) between January 2012 and May 2015. All

instrumentation used during the entire process, from initial coarse grinding to weighing on

the microbalance, was cleaned with an ethanol solution and allowed to dry after each

individual sample was processed to reduce cross sample contamination.

4.2.6. Calculation of Fertilizer N Recovery

The amount of total N in each component derived from the added fertilizer labeled

with 15N was calculated using a mass balance tracer technique that compared individual

ecosystem component 15N prior to and 1 year after N fertilization (Powlson and Barraclough

1993, Nadelhoffer and Fry 1994, Nadelhoffer et al. 1995).

Equation [4.1] calculated the individual system component N percent that originated

from the fertilizer N:

[4.1] %N from fertilizer in individual ecosystem component =

((δ15Nfinal - δ15Ninitial) / (δ15Nlabeled - δ15Ninitial)) * (100)

Equations [4.2] to [4.5] determined the percent recovery of fertilizer N for individual

ecosystem components:

[4.2] % 15N from labeled treatment in individual ecosystem component =

((δ15Nsample - δ15Npretreatment) / (δ15Nfertlizer - δ15Npretreatment)) * (100)

[4.3] Total N (g) = (Component dry weight * %N) / (100)

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[4.4] Fertilizer N in individual ecosystem components (mg)

( (Equation 4.3) * (Equation 4.4.) / (100) *1000)

[4.5] % N originated from fertilizer in individual ecosystem component =

[ (Equation 4.4) / (amount fertilizer applied)] *100

After individual ecosystem component fertilizer N recovery was determined, individual

component fertilizer N recovery was summed per plot to calculate total fertilizer N recovery

for the plot. The fertilizer N recovery value for the individual loblolly pine sampled in each

plot was multiplied by the number of loblolly pine trees in each individual plot to obtain the

total fertilizer N recovery for loblolly pine trees on an individual plot basis. The difference

between the amount of fertilizer N applied to the plot and the amount of fertilizer N

recovered after 1 growing season was considered lost from the system.


Total fertilizer N recovery, expressed as a percentage of fertilizer N applied, was

analyzed using a general linear model (GLM) analysis of variance (ANOVA) with Minitab®

17 (Minitab Inc., State College, Pennsylvania, USA). Percent data not normally distributed

was arcsin transformed prior to analysis. For Study 1, percent fertilizer N recovery (%) was

the response variable for the model, fertilizer treatment (CUF, NBPT, PCU, urea, control)

and season were fixed effects, and site was a random effect in the split plot analysis.

Individual ecosystem components were analyzed using a GLM ANOVA except the analysis

of individual mineral soil depth increments (0-15 cm, 15-30cm) which were analyzed as a

repeated measures ANOVA. For Study 2, percent fertilizer recovery (%) was the response

variable for the model, fertilizer treatment (CUF, NBPT, PCU, urea, control) was the fixed

effect, and site was a random effect in the analysis. Individual ecosystem components were

analyzed using a GLM ANOVA except the analysis of individual mineral soil depth

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increments (0-15 cm, 15-30cm) which were analyzed as a repeated measures ANOVA. To

determine if site factors (physiographic region, soil drainage, soil texture, etc.) were

correlated with total fertilizer recovery, a GLM ANOVA was also used for analysis. The

percent fertilizer recovery (%) was the response variable for the model, site factors

(physiographic region, soil drainage, soil texture, etc.) were fixed effects, and site was a

random effect in the analysis. All levels of significance were set at α = 0.05 and the P > |t|

values for the treatment means were tested. All post-hoc analysis was conducted with

Tukey’s HSD.

4.3. Results

4.3.1. Study 1 – 2011 Application – Spring vs. Summer

Nitrogen concentrations (g kg-1) in the foliage did not differ between application

season but were greater overall in the fertilized treatments (Table 4.4; In Appendix Figure

A.4.1., Figure A.4.2., Table A.4.1. to Table A.4.7., Table A.4.8) except for the CUF treatment.

Nitrogen concentrations in the foliage increased from 12.3 g kg-1 in the control to 14.1 g kg-1

in CUF (NS – not significant) and 14.4 g kg-1 to 15.0 g kg-1 in the other fertilizer treatments.

There was no difference between application season for N concentrations in the fine

branches, although N concentrations were greater in the fertilized treatments (6.6 g kg-1 to 7.4

g kg-1) compared to the control (4.9 g kg-1) with no differences among the EEFs. There were

no differences in N concentration between the control and the fertilizer treatments for any

other tree or soil component (Table 4.4) except in litterfall for the urea treatment (5.1 g kg-1),

and coarse roots in the urea (6.8 g kg-1) and PCU (7.0 g kg-1) treatments compared to the

unfertilized control (litterfall = 7.9 g kg-1, coarse roots = 8.8 kg g-1) plots.

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The 15N (‰) signature in all tree and soil components sampled was greater for the

fertilizer treatments compared to the control (Table 4.4). There were no differences in the 15N

values among the fertilizer treatments or between sampling dates for any components of the

loblolly pine trees. The 15N values were greatest in the foliage (83.9‰ to 97.1‰), fine

branches (74.3‰ to 86.9‰) and wood formed in the current growing season following

fertilization (59.3‰ to 73.8‰). The 15N in the current year litterfall was lower than foliage

(29.7‰ to 33.8‰). Wood formed in years prior to the fertilization treatments had lower 15N

(30.7‰ to 36.6‰), whereas the 15N was lowest in bark (15.0‰ to 18.9‰), fine roots (14.0‰

to 24.1‰) and coarse roots (8.5‰ to 15.8‰). In the soil, the 15N was greatest in the O

horizon (65.0‰ to 89.6‰), comparable to the 15N values in the foliage. Conversely, the 15N

signature in the surface 0-15 cm mineral soil in fertilized treatments ranged from 10.2‰ to

18.0‰ compared to the control (3.70‰). In the 15-30 cm mineral soil the 15N values ranged

from 8.5‰ to 11.9‰, with only the CUF treatment being significantly greater than the

control (5.44‰).

There were no differences in fertilizer N recovery in the individual components of

loblolly pine among the different fertilizer treatments (Table 4.4). Fertilizer N recovery in the

trees was greatest in the foliage, ranging from 9.1% to 14.8% of the N applied. Nitrogen

recovery in the other components of the trees was less, averaging 1.5% for fine branches,

4.3% for coarse branches, 1.0% in bark, 1.8% in wood formed in the current year after

fertilization and 3.9% in older wood. Fertilizer N recovery was higher in the fine roots,

averaging 11.1%, but less in the coarse roots which averaged 1.7%. Fertilizer N recovered in

the fresh litterfall was low, averaging 0.4%. In the soil, fertilizer N recovery in the O horizon

averaged 2.8% among the fertilizer treatments. In contrast, fertilizer N recovery in the surface

0-15 cm of the mineral soil was high, ranging from 13.4% in the urea treatment to 31.2% in

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the PCU treatment. Fertilizer N recover in the mineral soil from 15-30 cm was lower, ranging

from 5.1% to 8.3% among the fertilizer treatments.

There were no differences between the total ecosystem fertilizer N recovery between

the two seasons of application (Table 4.5). Total fertilizer N recovery differed among the N

fertilizer treatments for the combined spring and summer data with greater recovery for all

EEFs (CUF = 76.4%, NBPT = 80.6%, PCU = 76.2%) compared to urea (54.6%) (Figure 4.3)

but no differences among the EEFs. There were slight difference (NS) in the total fertilizer N

recovered in the tree, determined as the sum of N recovery in all aboveground and

belowground portions of the tree, between the treatments (CUF = 37.1%, NBPT = 44.7%,

PCU = 33.2%, urea = 33.8%) (Table 4.5, Table 4.8). Total fertilizer N recovery in the soil

was lower in the urea treatment (20.5%) compared to PCU (42.5%) and CUF (38.9%) but

there were no differences with NBPT (35.5%).

4.3.2. Study 2 – 2012 Application – Spring

Nitrogen concentrations (g kg-1) in the foliage increased (NS) from 12.5 g kg-1 in the

control to between 13.2 g kg-1 and 14.2 g kg-1 for fertilizer treatments (Table 4.6; In Appendix

Table A.4.7.). There was a difference in the N concentrations of fine branches between the

control (5.1 g kg-1) and CUF (7.3 g kg-1) the N concentration of bark between the control (2.1

g kg-1) and CUF (2.9 g kg-1), and for the N concentration in the wood produced for the year of

fertilization between the control (1.8 g kg-1) and CUF (2.9 g kg-1). There were no other

differences in N concentrations between the control and the fertilizer treatments, or among

fertilizer treatments, for any other tree, soil or other ecosystem component (Table 4.6).

The 15N (‰) signature in all of the tree and soil components sampled was greater in

the fertilizer treatments compared to the control, although there were no differences in the 15N

signatures among the individual fertilizer treatments (Table 4.6). For the loblolly pine trees,

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the 15N was greatest in the foliage (101.6‰ to 126.1‰), fine branches (98.7‰ to 111.8‰),

and wood produced in the current year after fertilization (71.9‰ to 88.1‰). The 15N in the

current year litterfall was lower than foliage (48.6‰ to 55.8‰). Wood formed in years prior

to fertilization had low 15N (34.8‰ to 40.3‰), whereas the 15N values were lowest in bark

(20.5‰ to 22.5‰), fine roots (31.2‰ to 36.1‰) and coarse roots (13.9‰ to 19.0‰). In the

understory, 15N values were high in both the sapling (59.6‰ to 83.3‰) and shrub (55.4‰ to

112.0‰) strata compared to the control (-0.24‰, -3.00‰ respectively). For the soil, there

were no differences among fertilizer treatments. The 15N was greatest in the O horizon for all

fertilizer treatments (55.2‰ to 91.1‰). The 15N signature for fertilizer treatments in the

surface 0-15 cm mineral soil ranged between 15.9‰ to 22.3‰ compared to the control

(3.27‰), whereas in the 15-30 cm mineral soil the 15N values ranged between 11.7‰ to

16.3‰ compared to 5.38‰ for the control (Table 4.6).

Differences did exist in fertilizer N recovery in the individual components of loblolly

pine among the different fertilizer treatments (Table 4.6). Fertilizer N recovery for loblolly

pine trees for all treatments was greatest in the foliage, ranging from 8.1% to 14.8% of the N

applied. More fertilizer N was recovered for foliage for NBPT (14.8%) compared to PCU

(8.1%) and fine branches (NBPT = 4.1%, PCU = 2.6%), with no other differences among

treatments for any other tree component. Fertilizer N recovery in the other components of the

tree was less, averaging 2.9% for coarse branches, 0.6% for bark, 2.2% for wood produced

the year of fertilization, and 4.0% for wood produced prior to fertilization (Table 4.6). There

were differences between fertilizer N recovery in fine roots between CUF (19.2%) and urea

(10.8%) but not for coarse roots (1.7% to 3.8%). The fertilizer N recovered in the litterfall

ranged from 1.4% to 1.8% with no differences among treatments. Fertilizer N recovery in the

soil ranged from 1.2% to 3.8% for the O horizon among treatments. The fertilizer N recovery

in the surface 0-15 cm of the mineral soil was high, ranging from 15.9% in the urea treatment

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to 24.2% in the PCU treatment. There were no differences in fertilizer N recovery for the 15-

30 cm mineral soil which ranged from 5.4% to 11.3% among the fertilizer treatments.

Total fertilizer N recovery differed among the fertilizer N treatments (Figure 4.4).

There was greater fertilizer N recovery in the EEFs (CUF 82.2%, NBPT 83.4%, PCU 80.7%)

compared to urea (66.2%) but no differences among the EEFs. There was a difference in the

total fertilizer N recovered in the tree, determined as the sum of N recovery in all

aboveground and belowground portions of the tree. For the tree, fertilizer N recovery was

greater in both CUF (47.0%) and NBPT (49.8%) compared to both PCU (38.5%) and urea

(34.8%), with no differences between CUF and NBPT or PCU and urea. For soil, PCU

(38.7%) was greater than urea (28.4%) (Table 4.7 and Table 4.8). For soil, determined as the

sum of N recovery in all both organic and mineral soil components, slight differences (NS)

occurred among treatments (CUF = 32.8%, NBPT = 30.6%, PCU = 38.7%, urea = 28.4%).

4.4. Discussion

Our study focused on assessing the differences in N uptake and ecosystem retention

following N fertilization in loblolly pine plantations of the southern United States. These

differences were assessed by determination of fertilizer N recovery among four different N

containing fertilizers and between different application seasons. By installing a study across

the natural range of loblolly pine we hoped to improve the understanding of fertilizer N

uptake efficiency in these systems through a regional synthesis, compared to a more detailed

analysis at a single site. This study tested the primary hypotheses: 1) whether differences

existed in the total fertilizer N recovery among conventional and enhanced efficiency

fertilizers; 2) if application season caused a difference in fertilizer N recovery among

individual fertilizer treatments; and 3) whether differences existed in the ecosystem

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partitioning of fertilizer N among conventional and enhanced efficiency fertilizers. Our

overall objective was to improve the fertilizer N use efficiency of southern loblolly pine

plantations.

Our first hypothesis that differences did not exist in total fertilizer N recovery among

individual fertilizer treatments (CUF, NBPT, PCU, urea) was rejected. Total fertilizer N

recovery was significantly greater for the enhanced efficiency fertilizers (CUF, NBPT, PCU)

compared to urea in both studies. Although significantly more fertilizer N was recovered for

enhanced efficiency fertilizers, differences generally did not exist among individual enhanced

efficiency fertilizers. The lower fertilizer N recovery for urea may be due to larger NH3

volatilization losses in the urea treatment compared to the EEFs.

In a companion study, Raymond et al. (2016) used sites from Study 1 and found NH3

volatilization losses following fertilization for urea ranged from 26% to 49% compared to

NH3 volatilization losses following fertilization with the same EEFs used in this research

ranged from 4% to 26%. Other studies have also reported large NH3 volatilization losses

following fertilization with urea (Kissel et al. 2004, Zerpa and Fox 2011). Elliot and Fox

(2014) found NH3 volatilization losses were approximately 50% less following fertilization in

loblolly pine plantations for EEFs compared to urea. When the mean percent NH3

volatilization loss reported by Raymond et al. (2016) are incorporated into the mass balance

for each respective treatment for both Study 1 and Study 2, the percent recovery of the

fertilizer N ranged from 90% to 100%. This evidence indicates that higher fertilizer N loss

through NH3 volatilization for urea compared to EEFs leads to a higher percent recovery of

fertilizer N for the entire ecosystem for the EEFs compared to urea.

Weather variables are often highly correlated with large NH3 volatilization losses

from urea, and include relative humidity exceeding a calculated critical relative humidity

(Elliot and Fox 2014, Raymond et al. 2016), high temperatures (Koelliker and Kissel 1988),

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high surficial wind speed (Kissel et al. 2004), and low precipitation (Kissel et al. 2004). In the

Raymond et al. (2016) companion study, days when the calculated critical relative humidity

exceeded the relative humidity was correlated with high NH3 volatilization losses for urea but

not for the EEFs. Site edaphic factors, such as the presence of an organic horizon (forest

floor) are also correlated with NH3 volatilization losses for urea application in forested

ecosystems (Cabrera et al. 2005, Kissel et al. 2009, Zerpa and Fox 2011). All sites for both

Study 1 and Study 2 had an O horizon of variable thickness and had a potential influence on

NH3 volatilization.

Although NH3 volatilization loss appears to be the primary loss mechanism in these

studies, leaching (Van Miegroet et al. 1994, Binkley et al. 1995, Lee and Jose 2005) and

denitrification (Shrestha et al. 2014) are also potential pathways for fertilizer N loss. We

indirectly tested the potential leaching loss of fertilizer N by sampling soil in the 15-30 cm

mineral soil and analyzing the sample for 15N. If this loss pathway was important large 15N

signatures would be present and the percent fertilizer N recovery would be significantly

different among treatments and the control, which did not occur. In the 15-30 cm mineral

soil, fertilizer N recovery ranged from 5.1% to 8.3% for Study 1 and 5.4% to 11.3% in Study

2. These results indicate that although some fertilizer N was moving deeper in soil profiles,

the percent loss for leaching was a minor loss pathway one year after fertilization. Another

potential loss pathway, denitrification, was not measured. Recent work examining

denitrification on similar sites found applied N losses through this pathway were low during

spring and summer months (Shrestha et al. 2014), and is typically considered a minor loss

pathway in aerobic forest soils (Bowden et al. 1991). Comparing N losses on well versus

poorly drained soils in our study found no differences in percent recovery of fertilizer N due

to soil drainage class (aerobic versus anaerobic systems), and this result indicates

denitrification is likely a minor loss mechanism. Although losses of fertilizer N through

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leaching and denitrification may have occurred, we concluded that these loss pathways were

minor in this study and NH3 volatilization was the primary loss pathway for fertilizer N from

these systems.

The second hypothesis that no differences occurred in fertilizer N recovery among

treatments between a spring and summer application was rejected. There was a higher

percentage of fertilizer N recovery in the ecosystem for urea in spring compared to summer,

whereas there were no differences between seasons for the EEFs. As previously stated, a

primary loss mechanism for applied urea N in loblolly pine plantations is NH3 volatilization

(Raymond et al. 2016), with fertilizer N loss generally correlated with weather variables

(Koelliker and Kissel 1988, Kissel et al. 2004, Elliot and Fox 2014, Raymond et al. 2016) and

their interaction with edaphic factors (Cabrera et al. 2005, Kissel et al. 2009, Zerpa and Fox

2011). High losses of fertilizer N from urea can occur regardless of application season

(Raymond et al. 2016). Although losses are generally higher in the summer under humid,

hotter conditions they can also be high in the winter or spring and highly dependent on

weather conditions at the time of fertilizer N application (Koelliker and Kissel 1988, Kissel et

al. 2004, Elliot and Fox 2014). Losses of fertilizer N were lower for EEFs compared to urea

(Raymond et al. 2016) for both seasons in Study 1 which translated to similar N recovery

rates for each EEF treatment between seasons. A likely explanation for the differences in

results between these two studies was Raymond et al. (2016) excluded roots to specifically

focus on loss mechanisms of fertilizer N. This study focused on ecosystem cycling of

fertilizer N and did not exclude roots for plant uptake. It is likely that plant uptake of fertilizer

N was rapid due to the presence of roots in the O horizon and upper mineral soil horizons

(Parker and Van Lear 1995, Adegbidi et al. 2004) and is a partial explanation of the small

increase in fertilizer N recovery for both seasons in Study 1 for urea.

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We rejected our third hypothesis that there was no difference in the ecosystem

partitioning of fertilizer N among treatments. When individual ecosystem components

(foliage, fine branches, O horizon, etc.) were analyzed separately, differences did occur

among fertilizer treatments for individual ecosystem.

Regardless of the season or treatment, the majority of the fertilizer N was recovered in

either the loblolly pine trees or soil. These results are similar to those of other tracer studies in

a variety of ecosystems (Nadelhoffer et al. 1995, 2004, Tietema et al. 1998, Burke et al. 2002,

Blanes et al. 2002). Yet the results for many of these studies indicate higher amounts of

applied N remaining in the soil and less N uptake for trees. Studies specific to southern

loblolly pine plantations indicate generally low fertilizer N uptake (<25%) by loblolly pine

trees (Baker et al. 1974, Mead and Pritchett 1975, Melin et al. 1983, Ballard 1984, Johnson

and Todd 1988, Johnson 1992, Li et al. 1999, Albaugh et al. 2004, Blazier et al. 2006). Yet

most studies in southern loblolly pine used urea as the fertilizer N source. In this study,

loblolly pine tree fertilizer N uptake of urea was 35% compared to 33% to 50% for all EEFs.

A clear trend exists for increased recovery of fertilizer N in loblolly pine trees for EEFs

compared to urea. Additionally, an increasing amount of fertilizer N was recovered with

EEFs in the soil for both studies (Study 1 = 30.5% to 38.7%, Study 2 = 35.5% to 42.5%)

compared to urea (Study 1 = 28.4%, Study 2 = 20.5%). These differences in the partitioning

of fertilizer N among ecosystem components between EEFs and urea is likely important in

the long term cycling of fertilizer N of loblolly pine plantations. Additional research will be

required to determine if this increase in fertilizer N retention for EEFs compared to urea

translates to an increase in FNUE for southern loblolly pine plantations.

Nitrogen containing fertilizers enriched with 15N can be used to integrate processes,

mechanisms and pathways of applied N in terrestrial ecosystems (Knowles and Blackburn

1993, Robinson 2001, Sulzman 2007) by improving the resolution of the amount of fertilizer

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N partitioned in ecosystem components. Fertilization studies in southern loblolly pine

plantations (Mead and Pritchett 1975, Albaugh et al. 1998, 2004, Jokela and Martin 2000,

Vogel and Jokela 2011, Carlson et al. 2014) have improved our understanding of loblolly

pine fertilizer response and provided a foundation in understanding the mechanisms and

pathways of applied N cycling in these systems. Yet these studies have generally been unable

to distinguish N originating from fertilizers compared to naturally existing N (Johnson 1992,

Fisher and Binkley 2000, Flint 2007, Kiser and Fox 2012). Differences in soil N between

control and fertilized plots in long term annual N fertilization experiments in loblolly pine

plantations is generally not detectable (Kiser and Fox 2012, Villanueva 2015), but using

fertilizers enriched with stable isotopes can improve the detection and resolution of

ecosystem partitioning and dynamics of N origination from fertilizers.

A primary benefit of this research has been the refinement of understanding

ecosystem fate of fertilizer N by using stable isotopes in mid-rotation southern loblolly pine

stands across an extensive geographic area. The results from this study encompass the entire

region where loblolly pine plantations are operationally fertilized and intensively managed,

and incorporates a large range of edaphic, climatic and stand variables. The results from this

study integrate factors across this entire region, and combined with other fertilizer N studies

in forest ecosystems focused on a smaller region or single site, improve the fundamental

understanding of the ecosystem partitioning of fertilizer N in southern loblolly pine

plantations.

4.5. Conclusions

The primary results for this research were: 1) significantly more fertilizer N was

recovered from the ecosystem for all enhanced efficiency fertilizers compared to urea; 2)

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differences occurred in the ecosystem partitioning of fertilizer N among treatments; 3) no

differences occurred in the ecosystem partitioning of fertilizer N among treatments between

seasonal fertilizer application; and 4) ecosystem fertilizer N recovery was independent of

edaphic, climatic or stand variables. The reduction of fertilizer N loss using enhanced

efficiency fertilizers compared to urea, regardless of the season of application, translates to an

increase in fertilizer N that remains in the system that is available for plant uptake. This

finding resulted in an increase in fertilizer nitrogen use efficiency for the enhanced efficiency

fertilizers compared to urea. Additional retention of fertilizer N in the soil for enhanced

efficiency fertilizers may also increase fertilizer nitrogen use efficiency into the stand rotation

if this fertilizer N becomes available to the plants, but additional research will be required for

determination.

These findings will help improve and refine management of southern loblolly pine

plantations. If economically viable, the use of enhanced efficiency fertilizers will enable

forest managers to apply fertilizer N during the spring or summer, providing a potentially

more synchronous timing for fertilizer application to seasonal plant demand. Overall, this

research provides a biological foundation for the effectiveness of using enhanced efficiency

fertilizers over the entire range where loblolly pine plantations are operationally viable in the

southern United States to improve fertilizer nitrogen use efficiency.

154

Acknowledgments


National Science Foundation Center for Advanced Forest Systems (CAFS) and the United

States Department of Agriculture National Institute of Food and Agriculture McIntire-Stennis

Capacity Grant. We thank David Mitchem for assistance in the laboratory, Andy Laviner and

Eric Carbaugh for field assistance and the numerous work study and seasonal student

employees for both field and laboratory assistance.

155

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164

Figures

Figure 4.1. Location map of loblolly pine stands in the southern United States selected to

evaluate ecosystem partitioning of fertilizer N following the application of urea or enhanced

efficiency N fertilizers enriched with 15N.

Study 1 – 2011

Study 2 - 2012

165

Figure 4.2. Plot sampling schematic for loblolly pine stands in the southern United States

selected to evaluate ecosystem partitioning of fertilizer N following application of urea or

enhanced efficiency N fertilizers enriched with 15N. Boxes below each category (crop tree,

understory, soils, litterfall) represent ecosystem components sampled.

166

Figure 4.3. The total fertilizer N recovery (% of fertilizer N applied) of the major ecosystem

components (tree - loblolly pine aboveground + belowground biomass, litterfall, and soil (O

horizon + mineral soil- 0 to 30cm)) for loblolly pine stands in the southern United States

selected to evaluate ecosystem partitioning of fertilizer N of urea or enhanced efficiency N

fertilizers enriched with 15N. Data represents combined fertilizer application dates for Study

1 for spring (March-April 2011) and summer (June 2011). Study 1 did not have a significant

understory category as in Study 2. Different letters represent significant differences at α =

0.05. Different letter fonts represent comparisons among treatments between same ecosystem

components (soil, litterfall, tree). The N application rate was 224 kg N ha-1. N =10.

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J.E. Raymond

167

Figure 4.4. The total fertilizer N recovery (% of fertilizer N applied) of the major ecosystem

components (Tree - loblolly pine aboveground + belowground biomass, understory, litterfall,

and soil (O horizon + mineral soil- 0 to 30cm)) for loblolly pine stands in the southern

United States selected to evaluate ecosystem partitioning of fertilizer N of urea or enhanced

efficiency N fertilizers enriched with 15N. Data represents fertilizer application dates for

Study 2 for spring (March-April 2012). Different letters represent significant differences at α

= 0.05. Different letter fonts represent comparisons among treatments between same

ecosystem components (soil, litterfall, understory, tree). The N application rate was 224 kg N

ha-1. N = 12.

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168

Tables

Table 4.1. Selected climate and physical characteristics of loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of

fertilizer N following application of urea or enhanced efficiency N fertilizers enriched with 15N. MAP = Mean annual precipitation. MAT = Mean annual

temperature.

Site

Latitude

Longitude

Altitude

(m)

MAP

(cm)

MAT

(°C)

Physiographic

Region

Soil Taxonomic

Class

Soil Series

Soil Drainage

Class

Study 1

VA1 37.440087 78.662396 60 109 13 Piedmont fine, mixed, subactive, mesic


SC1 33.869400 79.289300 2 125 17 Atlantic Coastal Plain fine, kaolinitic, thermic

Umbric Paleaquults Byars Very Poorly

AL1 33.233371 87.232384 48 125 17 Appalachian Plateau loamy-skeletal, mixed, subactive, thermic,

Typic Dystrudepts Montevallo Well

AL2 31.659464 86.272045 34 145 26 Southern Coastal Plain fine, smectitic, thermic,

Typic Hapludults Arundel Well

AR 33.422310 91.732651 21 140 17 Western Gulf Coastal

Plain fine-silty, mixed, active, thermic,

Typic Glossaqualfs Calhoun Poorly

Study 2

VA2 37.445331 78.662917 60 109 13 Piedmont fine, mixed, subactive, mesic


SC2 34.450000 80.505383 29 107 16 Sandhills thermic coated

Typic Quartzipsamments Lakeland Excessively

GA1 33.625317 82.801183 35 118 16 Piedmont fine kaolinitic thermic Rhodic Kandiudults

Davidson Well

GA2 31.339978 81.857283 1 114 18 Atlantic Coastal

Plain sandy siliceous thermic

Aeric Alaquods Leon Poorly

GA3 31.299333 81.847217 1 114 18 Atlantic Coastal Plain loamy, siliceous, subactive, thermic

Arenic Paleaquults Pelham Somewhat Poorly

FL 30.205267 83.866817 0.6 142 20 Eastern Gulf Coastal

Plain loamy siliceous superactive thermic

Aquic Arenic Hapludalfs Melvina Somewhat Poorly

MS 31.066717 89.602467 26 152 19 Western Gulf Coastal

Plain coarse loamy siliceous subactive thermic

Typic Paleudults Lucy Well

LA1 31.337017 93.182783 28 147 19 Western Gulf Coastal

Plain fine smectitic thermic Albaquic Hapludalfs

Corrigan Moderately Well

LA2 31.013333 93.422600 28 147 19 Western Gulf Coastal

Plain fine loamy siliceous subactive thermic

Plinthic Paleudults Malbis Well

J.E. Raymond

169

Site

Latitude

Longitude

Altitude

(m)

MAP

(cm)

MAT

(°C)

Physiographic

Region

Soil Taxonomic

Class

Soil Series

Soil Drainage

Class

Study 2

LA3 30.560533 90.727650 0.9 160 19 Western Gulf Coastal

Plain fine silty mixed active thermic

Typic Glossaqualfs Gilbert Poorly

OK 34.029333 94.825017 42 136 16 Western Gulf Coastal

Plain fine silty mixed active thermic Aquic

Paleudalfs Muskogee

moderately well

TX 31.13255 94.462533 32 127 20 Western Gulf Coastal

Plain fine loamy siliceous semiacitve thermic

Oxyaquic Glossudalfs Kurth moderately well

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Table 4.2. Characteristics of loblolly pine stands in the southern United States selected to evaluate ecosystem partitioning of fertilizer

N following application of urea or enhanced efficiency N fertilizers enriched with 15N.

Site Trees plot-1 Trees ha-1 Mean Height

(m)

Mean DBH

(cm)

Basal Area

(m2 ha-1)

Study 1

VA1 13 1260 13.1 16.4 27.4

SC1 5 540 15.7 25.9 29.4

AL1 4 400 19.2 24.4 19.0

AL2 6 600 18.1 20.5 20.5

AR 6 620 15.4 20.7 21.3

Study 2

VA2 8 880 9.1 15.1 16.1

SC2 15 1560 12.5 16.1 33.9

GA1 19 1800 7.2 8.6 10.6

GA2 15 1460 14.7 16.5 33.1

GA3 13 1340 13.6 15.1 25.0

FL 16 1580 10.2 13.4 23.2

MS 21 2160 12.5 13.8 35.4

LA1 7 720 14.9 19.3 21.5

LA2 6 780 14.8 17.5 18.7

LA3 23 2380 12.0 13.3 36.7

OK 15 1580 3.0 4.0 2.1

TX 13 1360 13.9 11.2 20.0

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Table 4.3. Selected physical and chemical soil characteristics of loblolly pine stands in the southern United States selected to

evaluate ecosystem partitioning of fertilizer N following application of urea or enhanced efficiency N fertilizers enriched with 15N.

Site Depth Bulk Density CEC pH P K Ca Mg Zn Mn Cu Fe B

Mehlich I Extracted

cm g cm-3 cmol kg-1 Water mg kg-1

Study 1

VA1 0-15 1.50 6.46 4.22 4.20 35.80 84.30 15.90 0.80 7.58 0.35 63.86 0.10

15-30 1.46 5.75 4.47 2.20 38.60 59.00 13.10 0.55 4.96 0.34 47.31 0.10

SC1 0-15 1.31 8.84 4.33 9.63 23.79 290.05 39.32 1.10 2.96 0.33 54.32 0.16

15-30 1.30 6.67 4.49 4.45 12.50 181.25 27.50 0.75 1.26 0.35 63.24 0.10

AL1 0-15 1.37 13.00 4.32 4.10 62.60 684.70 188.70 0.53 10.28 0.38 65.77 0.14

15-30 1.28 15.50 4.32 2.30 75.70 757.90 217.90 0.40 1.82 0.55 48.36 0.17

AL2 0-15 1.31 5.58 5.13 5.00 49.00 334.70 44.10 1.07 110.50 0.33 23.33 0.22

15-30 1.29 4.97 5.07 2.80 46.10 254.00 40.20 0.61 82.68 0.33 19.65 0.17

SC2 0-15 1.57 2.75 4.27 4.00 10.75 41.00 8.00 0.35 2.70 0.28 23.50 0.10

15-30 1.52 0.98 4.62 2.50 4.00 26.75 6.00 0.20 2.58 0.30 17.15 0.10

Study 2

VA2 0-15 1.47 6.99 4.45 3.90 39.40 135.50 21.20 1.01 13.84 0.36 96.24 0.10

15-30 1.43 6.10 4.53 2.80 45.80 64.00 15.70 0.81 7.27 0.35 38.30 0.10

GA1 0-15 1.41 6.14 5.48 3.56 53.11 552.56 123.78 0.78 185.53 1.09 24.08 0.20

15-30 1.36 5.69 5.19 2.80 43.30 408.30 150.60 0.36 123.85 0.96 18.27 0.15

GA2 0-15 1.23 5.90 3.74 4.78 11.56 70.11 19.56 0.34 0.46 0.28 11.30 0.10

15-30 1.32 4.59 4.02 6.11 6.78 47.67 11.22 0.22 3.81 0.27 12.59 0.10

GA3 0-15 1.23 6.34 3.73 3.00 10.13 70.75 19.88 0.36 0.49 0.21 15.45 0.10

15-30 1.32 4.99 3.98 4.44 7.00 49.89 11.89 0.29 0.41 0.17 13.67 0.10

FL 0-15 1.39 5.40 5.04 24.75 19.75 348.75 57.50 0.45 7.40 0.28 20.30 0.10

15-30 1.47 3.04 5.30 16.40 8.20 245.40 28.60 0.24 2.76 0.26 13.52 0.10

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172

Site Depth Bulk Density CEC pH P K Ca Mg Zn Mn Cu Fe B

Mehlich I Extracted

cm g cm-3 cmol kg-1 Water mg kg-1

Study 2

MS 0-15 1.44 5.28 4.99 6.25 19.75 235.50 26.00 0.38 25.15 0.35 19.00 0.10

15-30 1.45 2.96 4.89 2.20 15.20 59.00 14.60 0.24 26.10 0.40 14.26 0.10

LA1 0-15 1.37 4.78 4.46 2.40 22.80 138.80 49.00 1.00 9.94 0.78 66.48 0.10

15-30 1.36 6.86 4.53 1.40 25.20 148.80 96.20 0.48 3.78 0.82 37.98 0.10

LA2 0-15 1.48 3.32 4.74 2.00 16.00 130.80 20.60 0.30 27.16 0.26 37.32 0.10

15-30 1.37 2.03 4.79 1.25 12.50 93.75 23.75 0.18 14.65 0.28 25.85 0.10

LA3 0-15 1.48 5.48 4.43 3.44 16.78 215.11 57.33 0.78 73.88 0.29 110.42 0.11

15-30 1.48 6.69 4.46 1.78 14.67 174.22 83.11 0.36 23.99 0.29 71.69 0.10

AR1 0-15 1.21 4.85 5.00 3.05 27.53 331.42 73.84 0.62 178.62 0.50 43.21 0.13

15-30 1.38 5.06 4.86 2.05 22.95 230.42 65.53 0.55 138.03 0.53 33.30 0.10

OK 0-15 1.50 8.67 4.87 4.43 41.71 652.71 125.57 0.84 54.56 0.20 33.96 0.13

15-30 1.45 10.40 4.74 3.33 39.00 652.33 147.00 0.83 33.67 0.20 28.37 0.13

TX 0-15 1.50 5.55 4.67 2.75 26.75 217.25 63.25 0.40 10.73 0.36 39.36 0.11

15-30 1.57 6.51 4.62 2.00 26.50 209.00 101.25 0.26 7.11 0.41 30.64 0.13

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Table 4.4. The mean 15N (‰), N (g kg-1) and fertilizer N recovery (% of fertilizer N applied) for

individual ecosystem components for Study 1 (spring + summer 2011) for loblolly pine stands in

the southern United States selected to evaluate ecosystem partitioning of fertilizer N for urea or

enhanced efficiency N containing fertilizers enriched with 15N. Different letters represent

significant differences at α = 0.05. Numbers in parentheses represent the standard error of the

mean. The N application rate was 224 kg N ha-1. N = 10.

Spring + Summer Ecosystem

Component Treatment

15N

(‰)

N

(g kg-1)

Fertilizer N Recovery

(% fertilizer N applied)

Foliage

Control -3.23a (0.51) 12.3a (0.4) 0.0b CUF 96.5b (9.8) 14.1ab (0.5) 11.7a (2.1)

NBPT 97.1b (9.1) 15.0b (0.7) 14.8a (2.9) PCU 83.9b (9.3) 14.4b (0.4) 9.1a (0.9) Urea 92.7b (8.4) 14.7b (0.5) 12.2a (2.4)

Fine Branches

Control -3.01a (0.78) 4.9a (0.2) 0.0b CUF 86.9b (7.4) 7.4b (0.4) 1.7a (0.3)

NBPT 83.3b (6.7) 7.3b (0.5) 1.9a (0.4) PCU 74.3b (7.2) 6.9b (0.3) 1.1a (0.2) Urea 75.1b (8.9) 6.6b (0.4) 1.2a (0.3)

Coarse Branches

Control -2.58a (0.78) 3.4a (0.6) 0.0b CUF 58.4b (5.7) 3.3a (0.4) 4.6a (0.8)

NBPT 59.9b (6.3) 3.3a (0.3) 5.0a (1.0) PCU 52.8b (5.7) 3.3a (0.3) 3.5a (0.6) Urea 55.7b (3.1) 3.2a (0.2) 4.1a (0.6)

Bark


NBPT 18.9b (4.7) 2.5a (0.3) 1.1a (0.3) PCU 16.0b (2.8) 2.9a (0.5) 1.1a (0.3) Urea 15.0b (2.3) 2.3a (0.1) 0.7a ( 0.1)

Growth Ring (CGR) year of

sampling



Growth Rings (PGR)

composited prior to year of

sampling



Fine Roots (< 2mm)

Control 1.55a (1.89) 12.1a (0.8) 0.0b CUF 24.1b (3.5) 12.3a (0.6) 11.1a (1.8)

NBPT 19.8b (3.4) 12.8a (0.7) 12.6a (2.4) PCU 19.3b (3.2) 12.4a (0.6) 10.9a (1.3) Urea 14.0ab (1.5) 12.2a (0.6) 10.0a (1.8)

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Spring + Summer Ecosystem

Component Treatment

15N

(‰)

N

(g kg-1)


(% fertilizer N applied)

Coarse Roots (> 2mm)

Control 0.86a (0.73) 8.8a (0.6) 0.0b CUF 15.8c (2.3) 7.9ab (0.5) 2.0a (0.5)

NBPT 14.0bc (1.4) 7.7ab (0.5) 2.4a (0.8) PCU 12.2bc (1.8) 7.0b (0.2) 1.5a (0.5) Urea 8.5b (1.0) 6.8b (0.3) 0.8a (0.2)

Total Tree (Aboveground +

Belowground Biomass)

Control 0.0b CUF 37.1a

NBPT 44.7a PCU 33.2a Urea 33.8a

Litterfall (Current year)


NBPT 31.0b (7.5) 5.9ab (0.8) 0.4a (0.2) PCU 33.8b (8.2) 6.0ab (0.6) 0.5a (0.2) Urea 28.9b (6.9) 5.1b (0.7) 0.3a (0.1)

Organic Horizon

(Oi + Oe + Oa)


NBPT 71.5b (9.3) 8.6a (0.5) 2.8a (0.8) PCU 89.6b (7.8) 9.4a (0.8) 3.4a (1.0) Urea 65.0b (9.1) 8.5a (0.7) 2. 0a (0.4)

0-15 cm Mineral Soil

Control 3.70a (0.59) 1.0a (0.1) 0.0c CUF 16.6b (1.9) 1.2a (0.2) 27.8a (3.8)

NBPT 16.1b (1.7) 1.1a (0.2) 26.5a (4.2) PCU 18.0b (2.8) 1.3a (0.3) 31.2a (4.2) Urea 10.2b (1.5) 1.2a (0.3) 13.4b (3.7)



NBPT 10.3ab (1.0) 0.7a (0.2) 6.2a (1.3) PCU 11.2ab (1.5) 0.9a (0.3) 7.9a (1.7) Urea 8.5ab (0.6) 0.9a (0.3) 5.1a (2.1)

Total Soil (Organic + Mineral)

Control 0.0c CUF 38.9a

NBPT 35.5ab PCU 42.5a Urea 20.5b

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Table 4.5. Analysis of variance for treatment differences for fertilizer N recovery (% of fertilizer

N applied) for total and the primary ecosystem components for Study 1 (spring + summer 2011)

at mid-rotation loblolly pine plantations in the southern United States. N application rate was

224 kg N ha-1. N = 10.

Source Degrees of Freedom Adj. Sum of Squares Adj. Mean Squares F-Value P-

Value

Total Ecosystem Fertilizer N Recovery

Treatment 4 36904.2 9226.04 21.31 0.000 Season 1 82.5 82.52 0.19 0.665 Treatment*Season 4 416.0 104.00 0.24 0.914 Error 40 17317.7 432.94 Total 49 54720.4 Loblolly pine tree (foliage + fine branch + coarse branch + bark + CGR + PGR + fine roots + coarse roots)

Treatment 4 7433.0 1858.2 17.71 0.112 Season 1 760.2 760.2 1.25 0.150 Treatment*Season 4 400.4 100.1 0.95 0.443 Error 40 4196.7 104.9 Total 49 12790.4 Litterfall

Treatment 4 1.4512 0.36281 1.94 0.122 Season 1 0.3003 0.30035 1.61 0.212 Treatment*Season 4 0.1236 0.03090 0.17 0.955 Error 40 7.4662 0.18665 Total 49 9.3414 Soil (O horizon + 0-15 cm mineral soil + 15-30 cm mineral soil)

Treatment 4 6196.03 2065.34 6.91 0.001 Season 1 28.56 28.56 0.10 0.759 Treatment* Season

4 393.14 131.05 0.44 0.727

Error 40 9558.22 298.69 Total 49 Foliage

Treatment 4 1308 327.05 8.45 0.000

Error 45 1742 38.72

Total 49

Fine Branches

Treatment 4 21.20 5.3009 7.59 0.000

Error 45 31.45 0.6988

Total 49 52.65

Coarse Branches

Treatment 4 159.5 39.871 9.29 0.000

Error 45 193.0 4.290

Total 49

Bark

Treatment 4 8.199 2.0497 3.50 0.014

Error 45 26.327 0.5850

Total 49 34.526

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Source Degrees of Freedom Adj. Sum of Squares Adj. Mean Squares F-Value P-

Value

Current Growth Ring

Treatment 4 28.79 7.1981 8.06 0.000

Error 45 40.18 0.8930

Total 49 68.98

Previous Growth Rings

Treatment 4 152.2 38.054 6.57 0.000

Error 45 260.8 5.796

Total 49 413.1

Fine Roots (< 2 mm)

Treatment 4 655.8 163.94 4.23 0.005

Error 45 1742.3 38.72

Total 49

Coarse Roots (> 2 mm)

Treatment 4 36.07 9.017 3.85 0.009

Error 45 105.26 2.339

Total 49 141.33

Litterfall

Treatment 4 1.451 0.3628 2.07 0.101

Error 45 7.890 0.1753

Total 49 9.341

Organic Horizon

Treatment 4 69.57 17.393 3.79 0.010

Error 45 206.31 4.585

Total 49 275.88


Treatment 4 6732 1683.1 13.47 0.000

Error 45 5621 124.9

Total 49 12353


Treatment 4 442.3 110.59 2.76 0.039

Error 45 1802.5 40.06

Total 49 2244.8

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Table 4.6. The mean 15N (‰), N (g kg-1) and fertilizer N recovery (% of applied fertilizer N) for

individual ecosystem components for Study 2 (March-April 2012) for loblolly pine stands in the

southern United States selected to evaluate ecosystem partitioning of fertilizer N for urea or

enhanced efficiency N containing fertilizers enriched with 15N. Different letters represent

significant differences at α = 0.05. Numbers in parentheses represent the standard error of the

mean. The N application rate was 224 kg N ha-1. N = 12 for all ecosystem components except

sapling/pole and shrub strata where N = 7.

Ecosystem

Component Treatment

15N

(‰)

N

(g kg-1)


(% of applied N)

Foliage

Control -2.37a (0.51) 12.5a (0.4) 0.0c CUF 118.3b (11.2) 14.1a (0.8) 10.7ab (1.1)

NBPT 126.1b (8.5) 13.9a (0.6) 14.8b (1.8) PCU 101.6b (9.8) 13.2a (0.5) 8.1a (1.1) Urea 124.5b (11.2) 14.2a (0.5) 11.0ab (1.7)

Fine Branches

Control -2.02a (0.56) 5.1a (0.3) 0.0c

CUF 110.2b (10.2) 7.3b (0.6) 3.2ab (0.4) NBPT 111.8b (7.4) 6.7ab (0.3) 4.1b (0.5) PCU 98.7b (10.5) 6.1ab (0.5) 2.6a (0.5) Urea 108.4b (9.67) 6.2ab (0.4) 2.9ab (0.5)

Coarse Branches



Bark


NBPT 22.5b (2.1) 2.9b (0.1) 0.8a (0.2) PCU 20.5b (2.4) 2.4ab (0.1) 0.5a (0.1) Urea 20.8b (2.1) 2.5ab (0.2) 0.4a (0.1)

Growth Rings (CGR)- year of

sampling

Control -2.03a (0.44) 1.8a (0.1) 0.0b

CUF 80.0b (8.0) 2.9b (0.5) 2.2a (0.4) NBPT 88.1b (5.8) 2.7ab (0.1) 3.1a (0.6) PCU 71.9b (7.6) 2.3ab (0.1) 1.7a (0.3) Urea 78.9b (7.5) 2.3ab (0.1) 1.9a (0.3)

Growth Rings (PGR) -

composited prior to year of

sampling



Fine roots (< 2 mm)


NBPT 33.2b (3.8) 10.4a (0.4) 16.2ab (1.50) PCU 36.0b (3.5) 10.3a (0.5) 16.4ab (1.8) Urea 31.2b (5.1) 9.7a (0.5) 10.8b (1.4)

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Ecosystem

Component Treatment

15N

(‰)

N

(g kg-1)


(% of applied N)

Coarse roots (> 2 mm)



Total Tree (Aboveground + Belowground

Biomass)

Control 0.0c CUF 47.0a

NBPT 49.8a PCU 38.5b Urea 34.8b

Sapling/Pole

Stratum (n = 7)


NBPT 83.3b (21.3) 9.7a (2.8) 0.9a (0.5) PCU 64.1b (15.4) 8.2a (1.6) 0.3a (0.1) Urea 59.6ab (22.3) 8.1a (1.4) 0.9a (0.7)

Shrub Stratum (n = 7)


NBPT 112.0b (24.4) 9.4a (2.7) 0.7a (0.2) PCU 55.4ab (14.9) 9.0a (1.2) 1.4a (0.5) Urea 93.4b (18.3) 8.4a (0.7) 0.4a (0.2)

Litterfall



Organic horizon

(Oi + Oe + Oa)


NBPT 59.3b (5.8) 8.0a (0.6) 2.8a (1.3) PCU 91.1c (9.91) 8.7a (0.6) 3.8a (0.9) Urea 55.2b (6.34) 8.0a (0.7) 1.2a (0.4)

0-15 cm Mineral soil





NBPT 11.7ab (1.0) 0.4a (0.1) 5.4a (0.9) PCU 16.3b (3.0) 0.4a (0.0) 10.7a (3.4) Urea 15.2b (2.4) 0.4a (0.1) 11.3a (3.12)

Total Soil (Organic + Mineral)

Control 0.0b CUF 32.8a

NBPT 30.6a PCU 38.7a Urea 28.4a

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Table 4.7. Analysis of variance for treatment differences for fertilizer N recovery (% of fertilizer

N applied) for total and the primary ecosystem components for Study 2 (spring 2012) at mid-

rotation loblolly pine plantations in the southern United States. N application rate was 224 kg N

ha-1. N = 12 except for Understory where N = 7.

Source Degrees of

Freedom Adj. Sum of Squares Adj. Mean Squares F-Value P-Value

Total Ecosystem Fertilizer N Recovery

Treatment 4 61761 15440.4 87.77 0.000 Error 55 9676 175.9 Total 59 71437

Loblolly pine tree (foliage + fine branch + coarse branch + bark + CGR + PGR + fine roots + coarse roots

Treatment 4 19159 4789.75 77.09 0.000 Error 55 3417 62.13 Total 59 22576 Litterfall

Treatment 4 26.13 6.533 4.75 0.002 Error 55 75.60 1.375 Total 59 101.73

Understory (Saplings+Shrubs+Vines+Herbaceous) (n = 7)

Treatment 4 10.60 2.6489 4.03 0.010 Error 29 19.05 0.6568 Total 33 29.64 Soil (Organic Horizon + 0-15 cm mineral soil + 15-30 cm mineral soil)

Treatment 4 10823 2705.8 19.11 0.000 Error 55 7788 141.6 Total 59 18611 Foliage

Treatment 4 1472 367.93 18.36 0.000 Error 55 1102 20.04 Total 59

Fine Branches


Coarse Branches

Treatment 4 85.04 21.259 5.35 0.001 Error 55 218.62 3.975 Total 59

Bark


Current Growth Ring


Previous Growth Ring


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Source Degrees of

Freedom Adj. Sum of Squares Adj. Mean Squares F-Value P-Value

Fine Roots (< 2mm) Treatment 4 2751 687.81 26.99 0.000 Error 55 1401 25.48 Total 59 4153

Coarse Roots (> 2mm) Treatment 4 99.74 24.934 3.16 0.021 Error 55 434.48 7.900 Total 59 534.22

Litterfall Treatment 4 26.13 6.533 4.75 0.002 Error 55 75.60 1.375 Total 59 101.73

Organic Horizon (Oi + Oe + Oa) Treatment 4 115.2 28.79 2.54 0.050 Error 55 622.8 11.32 Total 59 738.0

0-15 cm Mineral Soil Treatment 4 4895 1223.7 9.74 0.000 Error 55 6910 125.6 Total 59 11805


181

Table 4.8. P > | t | values for fertilizer N treatment (CUF, NBPT, PCU, urea) contrasts for

total ecosystem fertilizer N recovery (% of fertilizer N applied) for Study 1 (spring + summer

2011) and Study 2 (spring 2012) at mid-rotation loblolly pine plantations in the southern

United States. The N application rate was 224 kg N ha-1. N =10.

Study 1- 2011 (n = 10) Study 2 – 2011 (n = 12) Differences of Fertilizer N Treatment Adj. P-Value Adj. P-Value

Total Ecosystem Fertilizer N Recovery Control – CUF 0.000 0.000

Control – NBPT 0.000 0.000 Control - PCU 0.000 0.000 Control - Urea 0.000 0.000 CUF - NBPT 0.903 0.975 CUF - PCU 1.000 0.795 CUF - Urea 0.008 0.004

NBPT - PCU 0.864 0.984 NBPT - Urea 0.001 0.020 PCU - Urea 0.010 0.054

Loblolly pine tree (foliage + fine branch + coarse branch + bark + CGR + PGR + fine roots + coarse roots

Control – CUF 0.000 0.000 Control – NBPT 0.000 0.000

Control - PCU 0.000 0.000

Control - Urea 0.000 0.000 CUF - NBPT 0.687 0.887 CUF - PCU 0.945 0.053 CUF - Urea 0.868 0.003

NBPT - PCU 0.261 0.007 NBPT - Urea 0.174 0.000 PCU - Urea 0.999 0.775

Litterfall



Control - Urea 0.683 0.683

CUF - NBPT 0.998 0.998

CUF - PCU 0.956 0.956

CUF - Urea 0.984 0.984

NBPT - PCU 0.995 0.995

NBPT - Urea 0.917 0.917

PCU - Urea 0.737 0.737 Soil (Organic Horizon + 0-15 cm mineral soil + 15-30 cm mineral soil)




NBPT - PCU 0.802 0.472 NBPT - Urea 0.141 0.992

PCU - Urea 0.010 0.238

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Chapter 5. Summary and Conclusions

5.1. Introduction

Optimizing forest plantations is critical to meet current and future societal demand for

wood products. Improving forest plantation productivity is essential to alleviate pressures to

harvest natural forests that function in providing clean water and air, wildlife habitat, and

biodiversity. The optimization of forest plantation productivity can be achieved through the

development and implementation of site specific management and includes the proper

identification and amelioration of nutrient deficiencies and imbalances to desired tree species.

In the southern United States, nitrogen (N) is generally the most limiting nutrient to growth

due to its disproportional requirement compared to other nutrients in photosynthesis.

Although fertilization with N (and phosphorous - P) has improved production in southern

plantation forests for decades, uncertainties exist to the pathways and mechanisms of applied

fertilizer N cycling in these systems. An improved understanding of the ultimate fate of

fertilizer N in loblolly pine (Pinus taeda L.) plantations is critical to improve site-specific

management for enhancing economic and environmental fertilizer N stewardship in southern

forest systems.

Improving the productivity of forest plantations is critical to meet the current and

projected global demand for traditional and emerging wood-based markets (FAO 2015). The

global optimization of forest plantations is critical to minimize the pressure on intensive

wood harvesting in natural forests which provide clean water and air, wildlife habitat, food,

carbon sequestration, and areas of biodiversity (Sedjo 1995, Fox 2000, Smethhurst 2014,

WWF 2015). Improved forest plantation productivity can be achieved through the

development and implementation of site specific forest management. The identification and

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amelioration of nutrient deficiencies and imbalances that limit tree growth is an integral

component of site specific forest management. A fundamental understanding of the processes

to improve nutrient availability and the cycling through plants and soils is critical to

developing globally sustainable forest management practices for both plantation and natural

forests.

In the southern United States, there are approximately 15.8 million ha of pine (Pinus

spp.) plantations (Wear and Greis 2012). Nitrogen and P are generally the most limiting

nutrients in the soils that support pine plantations in the southern United States. Both N and P

are required more than other nutrients due to the disproportional demand in photosynthesis

and foliar production (Chapin et al. 1986, Linder 1987, Vitousek and Howarth 1991). The

addition of N and P containing fertilizers since the 1950s has generally increased pine

plantation productivity in the southern United States by ameliorating these N and P nutrient

deficiencies (Allen 1987, Fischer and Binkley 2000, Richter et al. 2000, Allen et al. 2005,

Fox et al. 2007a, b). This finding has led to the annual N + P fertilization of approximately

one third of pine plantations in the South (Albaugh et al. 2004), with additional nutrients

applied when required on a site specific basis (Albaugh et al. 1998, Sampson and Allen 1999,

Jokela and Martin 2000, Vogel and Jokela 2011, Carlson et al. 2014).

Despite significant forest productivity gains from fertilization, uncertainty remains to

the ultimate fate of fertilizer N in these southern pine plantations (Fox et al. 2007a). For

example, some stands are nonresponsive to N fertilization because the fertilizer N may be: 1)

lost from the system via ammonia (NH3) volatilization, nitrification-denitrification, or

leaching; 2) immobilized in the soil by microbial communities or on soil exchange sites; 3)

partitioned into other ecosystem components such as understory competition; or 4) the site

may not be N deficient due to high rates of mineralization from decomposing organic matter

(Pritchett and Smith 1975, Birk and Vitousek 1986, Martin et al. 1999, Amateis et al. 2000,

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Albaugh et al. 2004, Carlson et al. 2014). Additionally, stands may not respond because

deficiencies of elements other than N create nutrient imbalances that limit growth (Vogel and

Jokela 2011, Carlson et al. 2014), and these sites may respond to N fertilization if the

required nutrients could be properly identified and effectively applied. The cumulative effect

of reduced fertilizer N uptake by trees translates to a lower fertilizer nitrogen use efficiency

(FNUE). An improved understanding of the pathways and mechanisms resulting in the

partitioning of fertilizer N in these pine plantations is needed to improve FNUE to: 1) reduce

negative environmental impacts; 2) improve productivity; and 3)reduce fertilizer inputs, all

critical issues concerning fertilizer with N in production agroecosystems today (Zhang et al.

2015). The inability to accurately determine site responsiveness of loblolly pine (Pinus taeda

L.) plantations to N fertilization hampers basic forest management decisions. Until the

understanding of the mechanisms and interactions governing fertilizer N cycling on a site-

specific basis improves, the ability to accurately predict sites responsive to N fertilization will

be limited.

The primary objective of this research was to improve the understanding N cycling

after fertilization in mid-rotation intensively managed loblolly pine plantations across the

entire range where loblolly pine is operationally viable in the southern United States.

Enhanced efficiency fertilizers were assessed to determine if there could be improvements to

FNUE compared to urea, the conventional form of fertilizer N used in southern forestry. In

Chapters 2, 3 and 4, the following questions were addressed: 1) Does fertilizer N loss,

specifically ammonia (NH3) volatilization, differ between conventional (urea) and enhanced

efficiency N containing fertilizers (see Chapter 2); 2) Does the use of enhanced efficiency N

containing fertilizers impact seasonal N uptake compared to urea of loblolly pines (see

Chapter 3); and 3) Is there a difference in the ecosystem partitioning of fertilizer N among

enhanced efficiency fertilizers and urea (see Chapter 4)? This research used N containing

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fertilizers enriched with the stable isotope 15N to improve the quantification and

understanding of fertilizer N cycling in loblolly pine plantations. The results from this

research provides forest managers the biological foundation to improve their decisions

concerning the economic feasibility of N fertilization, and whether using enhanced efficiency

N containing fertilizers may improve FNUE in mid-rotation loblolly pine plantations across

the entire southern United States.

This study encompassed diverse stand and site conditions across the primary

ecophysiographic regions of the southern United States where loblolly pine plantations are

operationally viable. Eighteen sites were selected from an existing network of forest thinning

and fertilization studies in mid-rotation loblolly pine plantations across the South for a spring

application of fertilizer N treatments. From these eighteen sites, a subset of six sites were

chosen for a summer application of fertilizer N treatments. Six sites were installed during

2011 and twelve sites in 2012. The six sites with a summer fertilization were installed in

2011. The same six sites installed in 2011 were chosen for both a spring and summer

fertilizer N application to assess fertilizer N loss and temporal N uptake of loblolly pine

5.2. Chapter Results and Discussion

The findings from this research highlight the ability to use enhanced efficiency

fertilizers to improve fertilizer nitrogen use efficiency compared to urea. There are three

primary conclusions from this research:

1) The enhanced efficiency fertilizers used in this research were equal in their ability

to reduce fertilizer N loss through ammonia (NH3) volatilization when compared

to urea, regardless of the timing of application. Although NH3 volatilization was

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positively correlated with certain weather variables for urea, correlations were

generally weak or negatively correlated with all enhanced efficiency fertilizers,

indicating fertilizer N loss from enhanced efficiency fertilizers was independent

from the weather conditions at the time of application (see Chapter 2).

2) The pattern of fertilizer N uptake by loblolly pines was similar for all fertilizer N

treatments after a spring and summer fertilization. One exception was the PCU

treatment, which had a lower N uptake pattern immediately following application, but

was similar to the other fertilizer treatments towards the middle and end of the

growing season. These results indicate that although release mechanisms and timing

may differ between enhanced efficiency fertilizers and urea, this does not adversely

affect the fertilizer N uptake of loblolly pine trees from EEFs compared to urea. Foliar

development occurred earlier and increased in total amounts (flushes) at the end of the

growing season for fertilized compared to unfertilized treatments. In addition, the

amount of N originating from both the fertilizer and natural system increased in

fertilized compared to unfertilized treatments (see Chapter 3).

3) More total fertilizer N was recovered from the entire ecosystem for all EEFs

compared to urea. There were differences between enhanced efficiency N containing

fertilizers and urea for certain ecosystem components, but few differences between

individual EEFs. Most fertilizer N recovered was either in the loblolly pine tree or

mineral soil, while other ecosystem components were minor sinks for the fertilizer N.

There were also differences between individual EEFs and urea for certain loblolly

pine tree components, indicating an increased FNUE for most EEFs compared to

urea. (see Chapter 4).

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An improved understanding of how conventional (urea) and enhanced efficiency

fertilizer N cycles through mid-rotation southern loblolly pine forests was provided by this

dissertation research. This research indicates enhanced efficiency N containing fertilizers can

improve fertilizer nitrogen use efficiency when compared to urea, in intensively managed

mid-rotation southern loblolly pine forests. Although there were clear differences between all

the enhanced efficiency N containing fertilizers and urea, there were not clear differences

between individual enhanced efficiency fertilizers.

When the results from individual Chapters are viewed collectively, it would appear

that the release mechanisms have a short term influence on the ecosystem partitioning of

fertilizer N. Urea rapidly releases N that is plant available, but the amount of urea-N lost

through NH3 volatilization can range from 5% to 90% and is generally dependent on weather

conditions at the time of application (Kissel et al. 2004, 2009, Elliot and Fox 2014, see

Chapter 2). Higher urea N losses after fertilization decrease stand FNUE for the stand. The

effects of elevated losses of fertilizer N in forest plantations are similar to agriculture, which

if inappropriately applied can create adverse environmental impacts, especially when viewed

in the context of the entire life cycle of urea production (Galloway et al. 2003, Zhang et al.

2015). Yet because N fertilization in forestry occurs only once or twice during an entire stand

rotation, adverse impacts associated with fertilizer N losses in forestry are primarily

economic due to reduced fertilizer response and the time required to carry the costs of

fertilization before obtaining a revenue stream (10 to 30 years, product dependent). This

contrasts to agriculture where crops, N fertilization, and revenue occurs annually, and the

environmental impacts associated with N fertilization have the potential to be more

problematic.

Although high fertilizer N losses after urea application may translate to a lower

fertilizer response for the stand, this response may not be immediately apparent. Although

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considerable urea N loss (26% to 49%) occurred after fertilization (see Chapter 2), fertilizer

N uptake of urea N by loblolly pine continued over the entire growing season regardless of

application season (see Chapter 3) which translated to a 30% to 35% fertilizer N uptake by

loblolly pine at the end of a single growing season (see Chapter 4). This percentage of

fertilizer N uptake by loblolly pine after urea fertilization was similar to findings in the

literature (Mead and Pritchett 1975, Albaugh et al. 1998, 2004, Blazier et al. 2006).

Conversely, fertilizer N loss after application of enhanced efficiency N containing fertilizers

(4% to 26%) was considerably lower than urea (see Chapter 2). Although fertilizer N uptake

patterns of most EEFs were similar to urea by loblolly pine over the entire growing season

regardless of application season (see Chapter 3), fertilizer N uptake ranged from 30% to 50%

for EEFs (see Chapter 4). Clearly and increase in FNUE occurred for EEFs compared to urea

in these mid-rotation southern loblolly pine forests. Additionally, fertilizer N remaining in the

soil was generally greater for EEFs (31% to 43%) compared to urea (21% to 28%). Future

research will need to determine whether additional gains in stand FNUE occur from N uptake

of fertilizer N that remained in the soil, or if the fertilizer N that remained in the soil is

immobilized and unavailable for plant uptake.

Although the findings of this research have addressed specific fundamental questions

concerning N cycling after fertilization in intensively managed mid-rotation loblolly pine

forests across the southern United States, key questions remain. Directions of future research

should include (Figure 5.1):

1) Loss mechanisms of fertilizer N during winter months. Although this research

focused on N fertilization in spring and summer, most forest fertilization in the

South occurs during winter months. Three questions specific to winter fertilization

with N are: 1) does a larger percentage of fertilizer N loss from urea occur through

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denitrification and/or leaching during the winter; 2) can enhanced efficiency

fertilizers reduce losses during a winter application period, and 3) does N uptake

by loblolly pines after a winter fertilization differ from spring and summer? This

question is especially important for bedded sites where 50% of the site may be

interbedded and inundated during the winter. Denitrification, as shown by

Shreshta et al. (2014) could be an increasingly important loss pathway for

fertilizer N after a winter fertilization with N.

2) Fertilizer application rates using enhanced efficiency fertilizers. This research has

shown that EEFs can reduce fertilizer N losses, translating to increased fertilizer N

remaining in the system. If fertilizer N application using EEFs could be reduced

between 20% to 50%, the amount of fertilizer N lost following urea fertilization,

could similar productivity gains occur with the current N fertilization rates of

urea?

3) Long term cycling of fertilizer N originating from enhanced efficiency fertilizers.

Because more fertilizer N remained in the loblolly pine system with EEFs

compared to urea, what does this mean? Do the incremental gains in fertilizer N

retention for the entire loblolly pine plantation ecosystem for EEFs compared to

urea translate to a sustained release of fertilizer N into the stand rotation? If yes,

does this continue to increase FNUE?

4) Productivity gains. Although there was a general trend in fertilizer N uptake for

the entire tree for the EEFs compared to urea, does this translate to an increase in

stand productivity, or is this luxury consumption for the trees with little gain in

productivity? This study was co-located with numerous fertilizer and thinning

trials, and the results from this research will need to be integrated with the results

from these existing fertilizer and thinning trials.

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The findings of this research provide forest managers the biological foundation of the

positive attributes in using enhanced efficiency fertilizers to improve fertilizer nitrogen use

efficiency in mid-rotation loblolly pine plantations across the southern United States when

compared to urea, the current primary sources of fertilizer N in forestry. Forest managers will

need to determine whether the positive attributes of enhanced efficiency fertilizers shown in

this study are economic viable within the constructs of their land management. Future

research will continue to refine the understanding of N cycling after fertilization with N

through the aforementioned recommended research directions to achieve sustained

incremental gains in productivity in loblolly pine plantations in the southern United States.

191

Figure 5.1. Implication and directions of current and future research for improving the

understanding of nitrogen cycling after fertilization with N containing fertilizers in mid-

rotation loblolly pine plantations across the southern United States.

192

Literature Cited Albaugh, T.J., H. L. Allen, P. M. Dougherty, and K. H. Johnsen. 2004. Long term growth



the southern United States from 1969 to 2004. South. J. Appl. For. 31(3):129-137. Allen, H.L. 1987. Forest fertilizers: nutrient amendment, and productivity, and environmental

impact. J. For. 85: 37-46. Amateis, R. L., J. Liu, M. J. Ducey and H. L. Allen. 2000. Modeling response to midrotation



loblolly pine stands. Ecology. 67(1):69-79. Cabrera, M.L., D.E. Kissel, N. Vaio, J. R. Craig, J.A. Rema, and L.A. Morris. 2005. Loblolly

pine needles retain urea fertilizer that can be lost as ammonia. Soil Sci. Soc. Am. J. 69(5):1525-1531

Carlson, C. A., T.R. Fox, H.L. Allen, T. J. Albaugh, R.A. Rubilar, J.L. Stape. 2014. Growth responses of loblolly pine in the southeast united states to midrotation applications of nitrogen, phosphorus, potassium, and micronutrients. For. Sci. 60(1):157-169.

Chapin, F.S., P.M. Vitousek, and K. Van Cleve. 1986. The nature of nutrient limitation in

plant-communities. Am Nat 127: 48–58. doi: 10.1086/284466. Elliot, J.R., and T.R. Fox. 2014. Ammonia volatilization following fertilization with urea or


Fisher, R.F., and D. Binkley. 2000. Ecology and Management of Forest Soils. John Wiley &

Sons. New York, NY. Fox, T.R. 2000. Sustained productivity in intensively managed forest plantations. For. Ecol.

Manage. 138:187-202. Fox, T.R., E.J. Jokela, and H.L. Allen. 2007a. The development of pine plantation silviculture

in the southern United States. J. For. 105(5):337-347. Fox, T.R., H. L. Allen, T. J. Albaugh, R. Rubilar, and C.A. Carlson. 2007b. Tree nutrition

and forest fertilization of pine plantations in the southern United States. South. J. Appl. For. 31(1):5-11.

Jokela, E.J. and T.A Martin. 2000. Effects of ontogeny and soil nutrient supply on production,


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Kissel, D., M. Cabrera, N. Vaio, J. Craig, J. Rema, and L. Morris. 2004. Rainfall timing and ammonia loss from urea in a loblolly pine plantation. Soil Sci. Soc. Am. J. 68:1744–1750. doi:10.2136/sssaj2004.1744


composition and ammonia loss from urea in a loblolly pine plantation. Soil Sci. Soc. Am. J. 73(2):630–637. doi:10.2136/sssaj2007.0377

Linder, S. 1987. Responses to water and nutrients in coniferous ecosystems. In: Potentials

and limitations of ecosystems analysis. Ecol. Studies 61, Schulze, E.D., and H.Z. Wolfer (eds.). Springer-Verlag, New York, NY. p. 180–202



Pritchett, W.L., W.H. Smith. 1972. Fertilizer response in young slash pine stands. Soil Sci.

Soc. Amer. J. 36(4): 660–663. Shrestha, R., B.D. Strahm, and E.B. Sucre. 2014. Nitrous oxide fluxes in fertilized Pinus

taeda plantations across a gradient of soil drainage classes. J. Envir. Qual. 43:1823–1832.

Vitousek, P.M., and P. A. Matson. 1985. Intensive harvesting and site preparation decrease

soil nitrogen availability in young plantations. South. J. Appl. For. 9:120-125. Vitousek, P. M., and R. W. Howarth. 1991. Nitrogen limitation on land and in the sea - how

can it occur? Biogeochemistry. 13:87–115. Vogel, J.G. and E.J. Jokela. 2011. Micronutrient limitations in two managed southern pine

stands planted on Florida Spodosols. Soil Science Society of America Journal. 75(3):1117-1124.

Wear, D.N., and J.G. Greis. 2012. The Southern Forests Futures Project: Summary Report.

Gen. Tech. Rep. SRS-168. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 13p.


plantations fertilized with urea in the southeast USA. Soil Sci. Soc. Am. J. 75:257–266. doi:10.2136/sssaj2010.0101.

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Appendix

195

Chapter 3

Figure A.3.1. The N concentration (g kg-1) values for flush 0, 1, and 2 for loblolly pine stands

in the southern United States selected to evaluate temporal uptake of fertilizer nitrogen

enriched with 15N following a spring and summer application of urea or enhanced efficiency

nitrogen fertilizers. The N application rate was 224 kg N ha-1. N = 6.

Spring Summer

1

Flush

0

Flush

1

Flush

2

196

Figure A.3.2. The 15N (‰) values for flush 0, 1, and 2 for loblolly pine stands in the southern

United States selected to evaluate temporal uptake of fertilizer nitrogen enriched with 15N

following a spring and summer application of urea or enhanced efficiency nitrogen

fertilizers. The N application rate was 224 kg N ha-1. N = 6.

Spring Summer

Flush

0

Flush

1

Flush

2

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Table A.3.1 P > | t | values for fertilizer N treatment (control, CUF, NBPT, PCU, urea)

contrasts for N concentration (g kg-1) between individual flushes (0, 1, 2) within sampling week

(6, 12, 18, 24, 30) for loblolly pine stands in the southern United States selected to evaluate

seasonal trends of fertilizer N uptake for urea or enhanced efficiency N containing fertilizers

enriched with 15N after a spring (2011) fertilization. Numbers in parentheses are the standard

error of the mean. The N application rate was 224 kg N ha-1. N = 6.

CUF Control NBPT PCU urea

Week-Flush p-value 6-0 6-1 1.0000 1.0000 1.0000 1.0000 1.0000 6-0 12-0 1.0000 1.0000 1.0000 1.0000 1.0000 6-0 18-0 1.0000 1.0000 1.0000 1.0000 1.0000 6-0 24-0 1.0000 1.0000 1.0000 1.0000 1.0000 6-0 30-0 1.0000 1.0000 1.0000 1.0000 1.0000

6-1 12-1 1.0000 1.0000 1.0000 1.0000 1.0000 6-1 18-1 0.5650 1.0000 0.9779 0.9918 1.0000 6-1 24-1 0.8915 1.0000 1.0000 1.0000 1.0000 6-1 30-1 1.0000 1.0000 1.0000 1.0000 1.0000

12-0 12-1 1.0000 1.0000 1.0000 1.0000 1.0000 12-0 12-2 1.0000 1.0000 1.0000 1.0000 1.0000 12-1 12-2 1.0000 1.0000 1.0000 1.0000 1.0000 12-0 18-0 1.0000 1.0000 1.0000 1.0000 1.0000 12-0 24-0 1.0000 1.0000 1.0000 1.0000 1.0000 12-0 30-0 1.0000 0.9996 1.0000 1.0000 1.0000 12-1 18-1 1.0000 1.0000 1.0000 1.0000 1.0000 12-1 24-1 1.0000 1.0000 1.0000 1.0000 1.0000 12-1 30-1 1.0000 1.0000 1.0000 1.0000 1.0000 12-2 18-2 1.0000 1.0000 1.0000 0.9990 1.0000 12-2 24-2 1.0000 1.0000 1.0000 0.9133 1.0000 12-2 30-2 1.0000 1.0000 1.0000 0.6253 1.0000 18-0 18-1 1.0000 1.0000 1.0000 1.0000 1.0000 18-0 18-2 1.0000 1.0000 1.0000 1.0000 1.0000 18-1 18-2 1.0000 1.0000 1.0000 1.0000 1.0000 18-0 24-0 1.0000 1.0000 1.0000 1.0000 1.0000 18-0 30-0 1.0000 1.0000 1.0000 1.0000 1.0000 18-1 24-1 1.0000 0.9956 1.0000 1.0000 1.0000 18-1 30-1 1.0000 1.0000 1.0000 1.0000 1.0000 18-2 24-2 1.0000 1.0000 1.0000 1.0000 1.0000 18-2 30-2 1.0000 1.0000 1.0000 1.0000 1.0000 24-0 24-1 1.0000 0.9996 1.0000 1.0000 1.0000 24-0 24-2 1.0000 0.9751 0.9972 1.0000 1.0000 24-1 24-2 1.0000 1.0000 1.0000 1.0000 1.0000 24-0 30-0 1.0000 1.0000 1.0000 1.0000 1.0000 24-1 30-1 1.0000 1.0000 1.0000 1.0000 1.0000 24-2 30-2 1.0000 1.0000 1.0000 1.0000 1.0000 30-0 30-1 1.0000 0.9952 1.0000 1.0000 1.0000 30-0 30-2 0.9793 0.6526 1.0000 0.9878 1.0000 30-1 30-2 1.0000 1.0000 1.0000 1.0000 1.0000

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Table A.3.2. P > | t | values for fertilizer N treatment (control, CUF, NBPT, PCU, urea)

contrasts for 15N values (‰) between individual flushes (0, 1, 2) within sampling week (6, 12, 18,

24, 30) for loblolly pine stands in the southern United States selected to evaluate seasonal trends

of fertilizer N uptake for urea or enhanced efficiency N containing fertilizers enriched with 15N

after a spring (2011) fertilization. Numbers in parentheses are the standard error of the mean.

The N application rate was 224 kg N ha-1. N = 6.


Week-Flush p-value 6-0 6-1 0.9524 1.0000 0.0136 1.0000 0.9993 6-0 12-0 0.9997 1.0000 1.0000 1.0000 1.0000 6-0 18-0 0.6136 1.0000 0.9891 0.2336 0.9024 6-0 24-0 1.0000 1.0000 0.9961 0.0600 0.4856 6-0 30-0 0.6275 1.0000 0.3106 0.0006 0.0004 6-1 12-1 0.9997 1.0000 1.0000 0.9994 0.9973 6-1 18-1 0.0438 1.0000 1.0000 0.0001 0.2945 6-1 24-1 0.9997 1.0000 1.0000 <.0001 0.7105 6-1 30-1 0.9116 1.0000 1.0000 <.0001 0.1212

12-0 12-1 0.9142 1.0000 0.1661 1.0000 0.8264 12-0 12-2 0.0286 1.0000 0.1176 0.9703 0.0599 12-1 12-2 0.9998 1.0000 1.0000 1.0000 1.0000 12-0 18-0 1.0000 1.0000 1.0000 0.9997 1.0000 12-0 24-0 1.0000 1.0000 1.0000 0.5962 0.9858 12-0 30-0 1.0000 1.0000 0.9980 0.6342 0.2959 12-1 18-1 0.9954 1.0000 1.0000 0.1319 1.0000 12-1 24-1 1.0000 1.0000 1.0000 0.0031 1.0000 12-1 30-1 1.0000 1.0000 1.0000 0.2753 1.0000 12-2 18-2 1.0000 1.0000 1.0000 0.9967 1.0000 12-2 24-2 1.0000 1.0000 1.0000 0.9996 1.0000 12-2 30-2 1.0000 1.0000 1.0000 0.0297 1.0000 18-0 18-1 0.6501 1.0000 0.7309 0.9963 0.9775 18-0 18-2 0.1804 1.0000 0.3676 0.8188 0.7544 18-1 18-2 1.0000 1.0000 1.0000 1.0000 1.0000 18-0 24-0 1.0000 1.0000 1.0000 1.0000 1.0000 18-0 30-0 1.0000 1.0000 1.0000 1.0000 1.0000 18-1 24-1 0.9999 1.0000 1.0000 1.0000 1.0000 18-1 30-1 1.0000 1.0000 1.0000 1.0000 1.0000 18-2 24-2 1.0000 1.0000 1.0000 1.0000 1.0000 18-2 30-2 1.0000 1.0000 1.0000 0.9993 1.0000 24-0 24-1 0.9590 1.0000 0.9999 1.0000 1.000 24-0 24-2 0.0438 1.0000 0.0176 0.0262 0.3980 24-1 24-2 1.0000 1.0000 0.9927 1.0000 1.0000 24-0 30-0 1.0000 1.0000 1.0000 1.0000 1.0000 24-1 30-1 1.0000 1.0000 1.0000 1.0000 1.0000 24-2 30-2 1.0000 1.0000 1.0000 0.9738 1.0000 30-0 30-1 0.9989 1.0000 0.9978 1.0000 1.0000 30-0 30-2 0.0790 1.0000 0.7525 0.1001 1.0000 30-1 30-2 1.0000 1.0000 1.0000 0.9873 1.0000

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contrasts for N concentration (g kg-1) between individual flushes (0, 1, 2) within sampling week

(6, 12, 18) for loblolly pine stands in the southern United States selected to evaluate seasonal

trends of fertilizer N uptake for urea or enhanced efficiency N containing fertilizers enriched

with 15N after a summer (2011) fertilization. Numbers in parentheses are the standard error of

the mean. The N application rate was 224 kg N ha-1. N = 6.


Week-Flush p-value 6-0 6-1 0.7654 1.0000 1.0000 1.0000 1.0000 6-0 6-2 0.9988 1.0000 1.0000 1.0000 1.0000 6-0 12-0 1.0000 1.0000 1.0000 1.0000 1.0000 6-0 18-0 1.0000 1.0000 1.0000 1.0000 1.0000 6-1 6-2 1.0000 1.0000 1.0000 1.0000 1.0000 6-1 12-1 1.0000 0.9998 1.0000 1.0000 1.0000 6-1 18-1 1.0000 1.0000 0.9996 1.0000 1.0000 6-2 12-2 1.0000 1.0000 1.0000 0.9614 1.0000 6-2 18-2 1.0000 1.0000 1.0000 0.9979 1.0000

12-0 12-1 0.7197 1.0000 1.0000 0.9989 0.9712 12-0 12-2 0.1430 1.0000 1.0000 0.9972 0.9163 12-0 18-0 1.0000 1.0000 1.0000 1.0000 1.0000 12-1 12-2 1.0000 1.0000 1.0000 1.0000 1.0000 12-1 18-1 1.0000 1.0000 1.0000 1.0000 1.0000 12-2 18-2 1.0000 1.0000 1.0000 1.0000 1.0000 18-0 18-1 0.9939 1.0000 1.0000 1.0000 0.9923 18-0 18-2 0.9431 1.0000 1.0000 0.6735 0.8790 18-1 18-2 1.0000 1.0000 1.0000 1.0000 1.0000

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contrasts for 15N values (‰) between individual flushes (0, 1, 2) within sampling week (6, 12, 18,

24, 30) for loblolly pine stands in the southern United States selected to evaluate seasonal trends

of fertilizer N uptake for urea or enhanced efficiency N containing fertilizers enriched with 15N

after a spring (2011) fertilization. Numbers in parentheses are the standard error of the mean.

The N application rate was 224 kg N ha-1. N = 6.


Week-Flush p-value 6-0 6-1 0.8369 1.0000 0.9992 1.0000 0.6771 6-0 6-2 0.1207 1.0000 0.4965 1.0000 1.0000 6-0 12-0 1.0000 1.0000 0.9900 0.9911 0.9995 6-0 18-0 0.6087 1.0000 0.2663 0.9517 0.9826 6-1 6-2 1.0000 1.0000 1.0000 1.0000 1.0000 6-1 12-1 1.0000 1.0000 0.9184 0.4330 1.0000 6-1 18-1 1.0000 1.0000 0.9999 0.4396 1.0000 6-2 12-2 1.0000 1.0000 0.1321 0.0351 0.9998 6-2 18-2 0.1242 1.0000 0.3964 0.0042 1.0000

12-0 12-1 0.9139 1.0000 0.9698 1.0000 0.9998 12-0 12-2 0.4816 1.0000 0.0033 0.7334 0.1299 12-0 18-0 1.0000 1.0000 1.0000 1.0000 1.0000 12-1 12-2 1.0000 1.0000 0.8689 1.0000 0.9969 12-1 18-1 1.0000 1.0000 1.0000 1.0000 1.0000 12-2 18-2 0.5059 1.0000 1.0000 1.0000 1.0000 18-0 18-1 1.0000 1.0000 1.0000 1.0000 0.9967 18-0 18-2 0.0298 1.0000 0.7010 0.8810 0.1391 18-1 18-2 0.4329 1.0000 0.9393 0.9997 0.9998

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Table A.3.5. P > | t | values for fertilizer N treatment (control, CUF, NBPT, PCU, urea) contrasts for mean foliar N concentration

(%) and mean foliar 15N (‰) for individual flushes (0, 1, 2) for individual sampling weeks (6, 12, 18, 24, 30) after a spring

fertilization (2011) for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced

efficiency N containing fertilizers enriched with 15N. The N application rate was 224 kg N ha-1. N = 6.

Week

6 12 18 24 30

Flush

0 1 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Contrasts N concentration (%)

Control-CUF 0.9999 1.0000 1.0000 1.0000 - 0.9999 0.9999 1.0000 0.9517 0.9913 0.8749 - 0.8072 1.0000 0.9960 - Control-NBPT 1.0000 1.0000 1.0000 0.9569 - 0.9977 0.6614 0.0359 0.9999 0.9994 0.2842 - 0.0212 0.2497 0.9935 - Control-PCU 1.0000 1.0000 1.0000 1.0000 - 1.0000 0.9942 1.0000 1.0000 1.0000 0.6509 - 0.8245 1.0000 0.9985 - Control-urea 1.0000 1.0000 1.0000 1.0000 - 1.0000 0.9931 1.0000 0.7392 0.9757 0.6864 - 0.0439 1.0000 0.9984 - CUF-NBPT 1.0000 0.9997 1.0000 1.0000 - 1.0000 1.0000 0.9996 1.0000 1.0000 1.0000 - 1.0000 0.9973 1.0000 - CUF-PCU 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 - CUF-urea 1.0000 0.9862 1.0000 1.0000 - 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 -

NBPT-PCU 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 0.9996 1.0000 1.0000 1.0000 - 1.0000 0.9995 1.0000 - NBPT-urea 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 - PCU-urea 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 -

15N (‰)

Control-CUF 0.6786 0.0002 0.0014 <.0001 - 0.0007 <.0001 <.0001 0.0168 <.0001 <.0001 - 0.0017 <.0001 <.0001 - Control-NBPT 0.9083 <.0001 0.1508 <.0001 - 0.0283 <.0001 <.0001 0.0187 <.0001 <.0001 - 0.0022 <.0001 <.0001 - Control-PCU 1.0000 1.0000 0.9983 0.2806 - 0.4625 <.0001 <.0001 0.1403 <.0001 <.0001 - 0.0546 <.0001 <.0001 - Control-urea 0.9373 0.0080 0.0598 <.0001 - 0.0137 <.0001 <.0001 0.0009 <.0001 <.0001 - <.0001 <.0001 <.0001 - CUF-NBPT 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 - CUF-PCU 0.9897 0.0621 0.8939 0.0414 - 1.0000 0.9512 0.9485 1.0000 1.0000 0.9875 - 1.0000 1.0000 1.0000 - CUF-urea 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 -

NBPT-PCU 0.9997 <.0001 1.0000 0.0793 - 1.0000 1.0000 1.0000 1.0000 1.0000 0.9448 - 1.0000 1.0000 1.0000 - NBPT-urea 1.0000 0.9907 1.0000 1.0000 - 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 - PCU-urea 0.9999 0.5909 1.0000 0.3810 - 1.0000 1.0000 1.0000 1.0000 1.0000 0.9944 - 1.0000 1.0000 1.0000 -

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Table A.3.6. P > | t | values for fertilizer N treatment (control, CUF, NBPT, PCU, urea) contrasts for the mean individual fascicle

mass (g) and mean N content (mg) for individual flushes (0, 1, 2) for individual sampling weeks (6, 12, 18, 24, 30) after a spring

fertilization (2011) for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced

efficiency N containing fertilizers enriched with 15N. The N application rate was 224 kg N ha-1. N = 6.

Week

6 12 18 24 30

Flush

0 1 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Contrasts Individual Fascicle Mass (g)

Control-CUF 0.6206 0.2566 0.1204 0.9018 0.7416 0.6295 0.0029 0.0181 0.8945 0.0043 0.0138 - 0.4461 0.0367 0.0003 - Control-NBPT 0.7835 0.2195 0.2138 0.5365 0.5525 0.9640 0.0028 0.0669 0.2996 0.0632 0.0072 - 0.5450 0.0027 0.0081 - Control-PCU 0.3507 0.5927 0.2545 0.3761 0.8515 0.2929 0.0027 0.1504 0.2785 0.0422 0.1543 - 0.7613 0.0063 0.1717 - Control-urea 0.8911 0.2023 0.1065 0.6407 0.8237 0.9165 0.0030 0.0320 0.1925 0.2153 0.0770 - 0.8272 0.0088 0.0037 - CUF-NBPT 0.8252 0.9215 0.7503 0.6205 0.7457 0.5979 0.9910 0.6601 0.3650 0.2952 0.8103 - 0.8750 0.3359 0.3087 - CUF-PCU 0.6588 0.5317 0.6718 0.4456 0.8371 0.1269 0.9820 0.3375 0.3409 0.3853 0.2829 - 0.6461 0.4978 0.0207 - CUF-urea 0.7199 0.8819 0.3744 0.7312 0.8727 0.7055 0.9910 0.9081 0.2410 0.0952 0.4687 - 0.3278 0.5751 0.4546 -

NBPT-PCU 0.5084 0.4696 0.9159 0.7880 0.5756 0.3139 0.9910 0.6347 0.9626 0.8571 0.1900 - 0.7625 0.7748 0.1851 - NBPT-urea 0.8903 0.9600 0.2294 0.8792 0.6071 0.8808 0.9820 0.7564 0.7876 0.5273 0.3356 - 0.4108 0.6864 0.7856 - PCU-urea 0.4249 0.4396 0.1917 0.6740 0.9598 0.2480 0.9730 0.4247 0.8239 0.4172 0.7250 - 0.6019 0.9062 0.5107 -

Individual Fascicle N content (mg)

Control-CUF 0.2760 0.4598 0.4359 0.6096 0.8296 0.1704 0.0013 0.0212 0.2376 0.0010 0.0068 - 0.0821 0.0502 0.0001 - Control-NBPT 0.4792 0.0887 0.7430 0.1137 0.5185 0.2843 0.0002 0.0031 0.8384 0.0201 0.0011 - 0.0153 <.0001 0.0018 - Control-PCU 0.3771 0.5126 0.5239 0.2508 0.8952 0.9541 0.0007 0.0879 0.9127 0.0312 0.0465 - 0.1676 0.0137 0.0514 - Control-urea 0.4868 0.0990 0.1945 0.2829 0.9188 0.3376 0.0007 0.0099 0.7243 0.0474 0.0248 - 0.1384 0.0046 0.0010 - CUF-NBPT 0.6985 0.3087 0.6502 0.2764 0.5983 0.7599 0.6016 0.4144 0.3273 0.2974 0.5430 - 0.4733 0.0229 0.4171 - CUF-PCU 0.8339 0.9298 0.8862 0.5187 0.6510 0.1886 0.8482 0.5301 0.2833 0.2231 0.4473 - 0.7133 0.5942 0.0423 - CUF-urea 0.6895 0.3362 0.5970 0.5690 0.8743 0.6754 0.8341 0.6858 0.4054 0.1628 0.6213 - 0.7921 0.3554 0.5441 -

NBPT-PCU 0.8588 0.2695 0.7560 0.6531 0.3071 0.3106 0.7405 0.1597 0.9248 0.8581 0.1736 - 0.2794 0.0778 0.2151 - NBPT-urea 0.9903 0.9544 0.3283 0.5993 0.4654 0.9098 0.7542 0.6925 0.8816 0.7194 0.2722 - 0.3276 0.1677 0.8369 - PCU-urea 0.8492 0.2945 0.5023 0.9392 0.7435 0.3672 0.9856 0.3175 0.8070 0.8567 0.7895 - 0.9173 0.6940 0.1493 -

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Table A.3.7. P > | t | values for fertilizer N treatment (control, CUF, NBPT, PCU, urea) contrasts for the fertilizer N (mg) in

individual fascicles (mg) for individual flushes (0, 1, 2) for week sampled (6, 12, 18, 24, 30) in loblolly pine stands of the southern

United States selected to evaluate fertilizer N uptake for urea or enhanced efficiency N containing fertilizers enriched with 15N after a

spring (2011) fertilization. Numbers in parentheses are standard error of mean. N application rate was 224 kg N ha-1. N = 6.

Week

Spring 6 12 18 24 30

Flush

0 1 0 1 2 0 1 2 0 1 2 3 0 1 2 3

Contrasts

Control-CUF 0.0914 0.0131 0.0013 <.0001 - 0.0088 <.0001 <.0001 0.0306 <.0001 <.0001 - 0.0184 <.0001 <.0001 - Control-NBPT 0.1666 <.0001 0.0073 <.0001 - 0.0286 <.0001 <.0001 0.0441 <.0001 <.0001 - 0.0030 <.0001 <.0001 - Control-PCU 0.8127 0.5196 0.1382 0.0064 - 0.1221 <.0001 0.0011 0.0854 <.0001 0.0009 - 0.0394 0.0003 <.0001 - Control-urea 0.1528 0.0056 0.0161 <.0001 - 0.0219 <.0001 <.0001 0.0223 <.0001 <.0001 - 0.0029 <.0001 <.0001 - CUF-NBPT 0.7515 0.0162 0.5421 0.2149 - 0.6447 0.8918 0.7727 0.8762 0.4131 0.2701 - 0.5125 0.1232 0.3026 - CUF-PCU 0.1442 0.0517 0.0623 0.0606 - 0.2606 0.2098 0.0872 0.6463 0.2839 0.1054 - 0.7552 0.6161 0.0784 - CUF-urea 0.7883 0.7420 0.3604 0.5082 - 0.7249 0.8157 0.0843 0.8970 0.4972 0.8087 - 0.5090 0.2708 0.7648 -

NBPT-PCU 0.2493 <.0001 0.2020 0.0026 - 0.5044 0.1653 0.0547 0.7616 0.7985 0.0075 - 0.3346 0.0426 0.4580 - NBPT-urea 0.9615 0.0356 0.7583 0.5581 - 0.9128 0.7121 0.6059 0.7755 0.8884 0.1798 - 0.9957 0.4294 0.4630 - PCU-urea 0.2304 0.0243 0.3301 0.0127 - 0.4375 0.3058 0.1614 0.5565 0.6925 0.1667 - 0.3320 0.2088 0.1420 -

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Table A.3.8. P > | t | values for fertilizer N treatments (control, CUF, NBPT, PCU, urea) contrasts for mean foliar N concentration (g

kg-1) and mean foliar 15N (‰) for individual flushes (0, 1, 2) for individual sampling weeks (6, 12, 18) after a summer fertilization

(2011) for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N

containing fertilizers enriched with 15N. The N application rate was 224 kg N ha-1. N = 6.

Week

6 12 18

Flush

0 1 2 0 1 2 3 0 1 2 3

Contrasts N concentration (g kg-1)

Control-CUF 0.9999 0.7097 0.9663 1.0000 0.4126 0.0726 - 1.0000 0.9977 1.0000 - Control-NBPT 0.8032 0.9212 0.6707 0.8559 0.6778 0.2368 - 1.0000 1.0000 0.9968 - Control-PCU 0.9975 1.0000 1.0000 1.0000 0.6772 0.7221 - 1.0000 1.0000 0.9998 - Control-urea 0.8549 0.9259 0.7479 1.0000 0.2016 0.0484 - 1.0000 0.9282 0.9921 - CUF-NBPT 1.0000 1.0000 1.0000 0.9945 1.0000 1.0000 - 1.0000 1.0000 1.0000 - CUF-PCU 1.0000 1.0000 0.9991 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 - CUF-urea 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 - NBPT-PCU 1.0000 1.0000 0.9282 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 - NBPT-urea 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 - PCU-urea 1.0000 1.0000 0.9582 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 -

15N (‰)

Control-CUF 0.1028 <.0001 <.0001 0.0002 <.0001 <.0001 - <.0001 <.0001 <.0001 - Control-NBPT 0.1540 0.0002 <.0001 <.0001 <.0001 <.0001 - <.0001 <.0001 <.0001 - Control-PCU 0.9407 0.8772 0.5334 0.0085 <.0001 <.0001 - 0.0432 0.0022 <.0001 - Control-urea 0.3323 <.0001 <.0001 0.0006 <.0001 <.0001 - 0.0028 <.0001 <.0001 - CUF-NBPT 1.0000 1.0000 1.0000 1.0000 1.0000 0.9912 - 1.0000 1.0000 1.0000 - CUF-PCU 1.0000 0.0879 0.0156 1.0000 0.9848 1.0000 - 0.9994 0.9945 0.2432 - CUF-urea 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 - 1.0000 1.0000 1.0000 - NBPT-PCU 1.0000 0.6456 0.1769 1.0000 0.9481 0.3059 - 0.9953 0.9999 0.9634 - NBPT-urea 1.0000 1.0000 1.0000 1.0000 1.0000 0.9997 - 1.0000 1.0000 1.0000 - PCU-urea 1.0000 0.1723 0.1149 1.0000 1.0000 1.0000 - 1.0000 0.9976 0.9985 -

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Table A.3.9. P > | t | values for fertilizer N treatment (control, CUF, NBPT, PCU, urea) contrasts for the mean individual fascicle

mass (g) and N content (mg) for individual flushes (0, 1, 2) for individual sampling weeks (6, 12, 18) after a summer fertilization

(2011) for loblolly pine stands in the southern United States selected to evaluate fertilizer N uptake of urea or enhanced efficiency N

containing fertilizers enriched with 15N. The N application rate was 224 kg N ha-1. N = 6.

Week

6 12 18

Flush

0 1 2 0 1 2 3 0 1 2 3

Contrasts Individual Fascicles Mass (g)

Control-CUF 0.4466 0.0959 <.0001 0.3709 0.1007 0.1343 - 0.9959 0.0419 0.7713 - Control-NBPT 0.7373 0.0002 <.0001 0.7464 0.0151 0.0146 - 0.9171 0.0978 0.0176 - Control-PCU 0.4095 0.0190 0.0009 0.9868 0.1085 0.1496 - 0.5306 0.0537 0.2212 - Control-urea 0.7763 0.0006 0.0009 0.8001 0.0089 0.0080 - 0.2728 0.0090 0.0062 - CUF-NBPT 0.6694 0.0283 0.9833 0.2249 0.4060 0.3220 - 0.9212 0.6941 0.0359 - CUF-PCU 0.9488 0.4760 0.2463 0.3797 0.9701 0.9539 - 0.5272 0.9125 0.3491 - CUF-urea 0.6326 0.0572 0.2593 0.2523 0.3041 0.2248 - 0.2750 0.5405 0.0137 - NBPT-PCU 0.6234 0.1318 0.2548 0.7340 0.3854 0.2950 - 0.4650 0.7768 0.2351 - NBPT-urea 0.9591 0.7599 0.2681 0.9440 0.8424 0.8204 - 0.3203 0.3160 0.6967 - PCU-urea 0.5877 0.2272 0.9746 0.7874 0.2871 0.2039 - 0.0871 0.4706 0.1166 -

Individual Fascicle N Content (mg)

Control-CUF 0.1263 0.0021 0.0001 0.7462 0.0042 0.0244 - 0.7193 0.0148 0.0403 - Control-NBPT 0.0497 0.0001 <.0001 0.0105 0.0017 0.0007 - 0.4571 0.0623 0.0060 - Control-PCU 0.0752 0.0248 0.0419 0.5903 0.0105 0.0230 - 0.8285 0.0725 0.0117 - Control-urea 0.0576 0.0003 0.0005 0.4643 0.0003 0.0004 - 0.2517 0.0026 0.0019 - CUF-NBPT 0.6533 0.4052 0.6687 0.0508 0.7649 0.2128 - 0.7104 0.5438 0.0495 - CUF-PCU 0.7958 0.3700 0.0540 0.3897 0.7395 0.9811 - 0.5650 0.4980 0.4456 - CUF-urea 0.7015 0.5562 0.7016 0.2926 0.4001 0.1488 - 0.4289 0.5260 0.0195 - NBPT-PCU 0.8488 0.0866 0.0197 0.2645 0.5281 0.2215 - 0.3451 0.9434 0.2206 - NBPT-urea 0.9473 0.8062 0.4183 0.3555 0.5864 0.8395 - 0.6735 0.2168 0.6929 - PCU-urea 0.9009 0.1400 0.1194 0.8459 0.2420 0.1554 - 0.1735 0.1921 0.1075 -

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Chapter 4

Figure A.4.1. The fertilizer N recovery (% of fertilizer N applied) of the major ecosystem

components (loblolly pine aboveground biomass, loblolly pine roots, litterfall, O horizon,

mineral soil) for loblolly pine stands in the southern United States selected to evaluate ecosystem

partitioning of fertilizer N of urea or enhanced efficiency N fertilizers enriched with 15N. Data

represents spring application date (March-April 2011) for Study 1. Different letters represent

significant differences at α = 0.05. The N application rate was 224 kg N ha-1. N = 5.

ab

a

ab

b

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Figure A.4.2. The fertilizer N recovery (% of fertilizer N applied) of the major ecosystem

components (loblolly pine aboveground biomass, loblolly pine roots, litterfall, O horizon,

mineral soil) for loblolly pine stands in the southern United States selected to evaluate ecosystem

partitioning of fertilizer N of urea or enhanced efficiency N fertilizers enriched with 15N. Data

represents summer application date (June 2011) for Study 1. Different letters represent

significant differences at α = 0.05. The N application rate was 224 kg N ha-1. N=5.

ab

a a

b

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Table A.4.1. Analysis of variance for treatment differences for individual component fertilizer N

recovery (% of fertilizer N applied) for Study 1 (spring 2011) at mid-rotation loblolly pine

plantations in the southern United States. N application rate was 224 kg N ha-1. N = 10.

Source Degrees of Freedom Adj. Sum of Squares Adj. Mean Squares F-Value P-Value

Total Ecosystem Fertilizer N Recovery Treatment 4 19094 4773.6 12.52 0.000 Error 20 7624 381.2 Total 24 26718

Loblolly pine tree (foliage + fine branch + coarse branch + bark + CGR + PGR + fine roots + coarse roots Treatment 4 5413 1353.2 11.31 0.000 Error 20 2392 119.6 Total 24 7805 Soil (Organic Horizon + 0-15 cm mineral soil + 15-30 cm mineral soil) Treatment 4 4569 1142.3 7.46 0.001 Error 20 3064 153.2 Total 24 7633 Foliage Treatment 4 880.4 220.09 5.05 0.006 Error 20 871.0 43.55 Total 24

Fine Branches Treatment 4 14.87 3.7178 4.72 0.008 Error 20 15.76 0.7880 Total 24 30.63

Coarse Branches Treatment 4 154.98 38.745 12.76 0.000 Error 20 60.71 3.035 Total 24

Bark Treatment 4 5.263 1.3158 1.93 0.145

Error 20 13.641 0.6821 Total 24 18.904

Current Growth Ring Treatment 4 15.31 3.8272 6.16 0.002 Error 20 12.43 0.6217 Total 24 27.74

Previous Growth Rings Treatment 4 100.8 25.188 4.87 0.007 Error 20 103.4 5.172 Total 24 204.2

Fine Roots (< 2 mm) Treatment 4 638.7 159.66 5.38 0.004 Error 20 593.9 29.69 Total 24 1232.5

Coarse Roots (> 2 mm) Treatment 4 33.46 8.364 2.15 0.112

Error 20 77.72 3.886 Total 24 111.18

Litterfall Treatment 4 1.054 0.2634 0.93 0.468

Error 20 5.684 0.2842 Total 24 6.737

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Organic Horizon (Oi + Oe + Oa) Treatment 4 26.08 6.519 1.86 0.158

Error 20 70.24 3.512 Total 24 96.32


15-30 cm Mineral Soil Treatment 4 130.7 32.67 3.03 0.042 Error 20 215.5 10.77 Total 24 346.2

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Table A.4.2. Analysis of variance for treatment differences for individual component fertilizer N

recovery (% of fertilizer N applied) for Study 1 (summer 2011) at mid-rotation loblolly pine

plantations in the southern United States. N application rate was 224 kg N ha-1. N = 10.


Total Ecosystem Fertilizer N Recovery Treatment 4 18226 4556.5 9.40 0.000 Error 20 9694 484.7 Total 24 27920

Loblolly pine tree (foliage + fine branch + coarse branch + bark + CGR + PGR + fine roots + coarse roots Treatment 4 2420 605.10 6.71 0.001 Error 20 1804 90.22 Total 24 4225 Soil (Organic Horizon + 0-15 cm mineral soil + 15-30 cm mineral soil) Treatment 4 8243 2060.8 8.43 0.000 Error 20 4887 244.3 Total 24 13130 Foliage Treatment 4 466.2 116.56 3.12 0.038 Error 20 746.8 37.34 Total 24

Fine Branches Treatment 4 7.269 1.8173 2.66 0.063 Error 20 13.655 0.6827 Total 24 20.924

Coarse Branches Treatment 4 60.63 15.159 4.94 0.006 Error 20 61.35 3.067 Total 24

Bark Treatment 4 4.851 1.2128 2.32 0.092

Error 20 10.464 0.5232 Total 24 15.315

Current Growth Ring Treatment 4 14.42 3.604 2.71 0.059

Error 20 26.57 1.328 Total 24 40.99

Previous Growth Ring Treatment 4 77.85 19.462 3.04 0.041 Error 20 128.01 6.400 Total 24 205.86

Fine Roots (< 2 mm) Treatment 4 196.3 49.09 1.01 0.424

Error 20 968.4 48.42 Total 24 1164.7

Coarse Roots (< 2 mm) Treatment 4 9.520 2.3799 3.97 0.016 Error 20 11.995 0.5997 Total 24 21.514

Litterfall

Treatment 4 0.5212 0.13031 1.46 0.251

Error 20 1.7824 0.08912 Total 24 2.3037

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Organic Horizon Treatment 4 50.54 12.636 2.04 0.127

Error 20 123.69 6.184 Total 24 174.23


15-30 cm Mineral Soil Treatment 4 354.5 88.63 1.22 0.335

Error 20 1458.6 72.93 Total 24 1813.1

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Table A.4.3. P > | t | values for fertilizer N treatment (CUF, NBPT, PCU, urea) contrasts for

primary individual component fertilizer N recovery (% of fertilizer N applied) for Study 1 (spring

and summer 2011) at mid-rotation loblolly pine plantations in the southern United States. The N

application rate was 224 kg N ha-1. N =5.

Spring 2011 Summer 2011 Differences in Fertilizer

N Treatments Adjusted P-Value Adjusted P-Value

Total Ecosystem Fertilizer N Recovery Control – CUF 0.000 0.001

Control – NBPT 0.000 0.001

Control – PCU 0.000 0.000


CUF - NBPT 1.000 0.997 CUF - PCU 0.967 0.972 CUF - Urea 0.322 0.536


PCU - Urea 0.682 0.231

Loblolly Pine Tree Control – CUF 0.000 0.006




CUF - NBPT 0.746 0.973 CUF - PCU 0.934 0.999 CUF - Urea 0.757 1.000 NBPT - PCU 0.307 0.915 NBPT - Urea 0.157 0.952 PCU - Urea 0.994 1.000 Litterfall Control – CUF 0.673 0.643 Control – NBPT 0.661 0.307 Control – PCU 0.387 0.329 Control - Urea 0.787 0.961 CUF - NBPT 1.000 0.973 CUF - PCU 0.987 0.980 CUF - Urea 1.000 0.951


PCU - Urea 0.956 0.712

Soil Control – CUF 0.001 0.005





PCU-Urea 0.462 0.039

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individual component fertilizer N recovery (% of fertilizer N applied) for Study 1 (spring +

summer 2011) at mid-rotation loblolly pine plantations in the southern United States. The N

application rate was 224 kg N ha-1. N =10.

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Foliage Current Growth Ring Litterfall Control – CUF 0.001 Control – CUF 0.002 Control – CUF 0.332

Control – NBPT 0.000 Control – NBPT 0.000 Control – NBPT 0.190

Control – PCU 0.017 Control – PCU 0.003 Control – PCU 0.083

Control - Urea 0.001 Control - Urea 0.000 Control - Urea 0.657

CUF - NBPT 0.795 CUF - NBPT 0.981 CUF - NBPT 0.998 CUF - PCU 0.880 CUF - PCU 1.000 CUF - PCU 0.951 CUF - Urea 1.000 CUF - Urea 0.835 CUF - Urea 0.982

NBPT - PCU 0.255 NBPT - PCU 0.950 NBPT - PCU 0.994 NBPT - Urea 0.875 NBPT - Urea 0.988 NBPT - Urea 0.908

PCU - Urea 0.802 PCU - Urea 0.745 PCU - Urea 0.714

Fine Branches Previous Growth Ring Organic Horizon Control – CUF 0.000 Control – CUF 0.023 Control – CUF 0.043







Coarse Branches Fine Roots (< 2 mm) 0-15 cm Mineral Soil Control – CUF 0.000 Control – CUF 0.024 Control – CUF 0.000







Bark Coarse Roots (> 2 mm) 15-30 cm Mineral Soil Control – CUF 0.102 Control – CUF 0.039 Control – CUF 0.041







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individual component fertilizer N recovery (% of fertilizer N applied) for Study 1 (spring 2011)

at mid-rotation loblolly pine plantations in the southern United States. The N application rate

was 224 kg N ha-1. N =5.

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Foliage Current Growth Ring Litterfall Control – CUF 0.029 Control – CUF 0.013 Control – CUF 0.673 Control – NBPT 0.004 Control – NBPT 0.003 Control – NBPT 0.661 Control – PCU 0.170 Control – PCU 0.014 Control – PCU 0.387 Control - Urea 0.028 Control - Urea 0.006 Control - Urea 0.787 CUF - NBPT 0.896 CUF - NBPT 0.963 CUF - NBPT 1.000 CUF - PCU 0.891 CUF – PCU 1.000 CUF - PCU 0.987 CUF - Urea 1.000 CUF – Urea 0.998 CUF - Urea 1.000


PCU - Urea 0.888 PCU – Urea 0.996 PCU - Urea 0.956

Fine Branches Previous Growth Ring Organic Horizon Control – CUF 0.017 Control – CUF 0.021 Control – CUF 0.231 Control – NBPT 0.007 Control – NBPT 0.018 Control – NBPT 0.573 Control – PCU 0.322 Control – PCU 0.025 Control – PCU 0.129 Control - Urea 0.164 Control - Urea 0.617 Control - Urea 0.477 CUF - NBPT 0.993 CUF - NBPT 1.000 CUF - NBPT 0.961 CUF - PCU 0.552 CUF – PCU 1.000 CUF - PCU 0.997 CUF - Urea 0.794 CUF – Urea 0.309 CUF - Urea 0.986






Control - Urea 0.015 Control - Urea 0.053 Control - Urea 0.157 CUF - NBPT 0.601 CUF - NBPT 0.443 CUF - NBPT 0.890 CUF - PCU 0.128 CUF - PCU 0.796 CUF - PCU 0.999 CUF - Urea 0.501 CUF - Urea 0.993 CUF - Urea 0.380




Control – NBPT 0.238 Control – NBPT 0.107 Control – NBPT 0.097 Control – PCU 0.371 Control – PCU 0.352 Control – PCU 0.131 Control - Urea 0.696 Control - Urea 0.950 Control - Urea 0.431 CUF - NBPT 0.996 CUF - NBPT 0.934 CUF - NBPT 0.988 CUF - PCU 0.962 CUF - PCU 1.000 CUF - PCU 0.964 CUF - Urea 0.740 CUF - Urea 0.800 CUF - Urea 0.633



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Table A.4.6. P > | t | values for treatment (CUF, NBPT, PCU, urea) contrasts for individual

ecosystem component fertilizer N recovery (% of fertilizer N applied) for Study 1 (summer 2011)


was 224 kg N ha-1.

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Foliage Current Growth Ring Litterfall Control – CUF 0.121 Control – CUF 0.238 Control – CUF 0.643 Control – NBPT 0.033 Control – NBPT 0.173 Control – NBPT 0.307 Control – PCU 0.232 Control – PCU 0.308 Control – PCU 0.329 Control - Urea 0.077 Control - Urea 0.036 Control - Urea 0.961 CUF - NBPT 0.965 CUF - NBPT 1.000 CUF - NBPT 0.973 CUF - PCU 0.995 CUF – PCU 1.000 CUF - PCU 0.980 CUF - Urea 0.999 CUF – Urea 0.855 CUF - Urea 0.951


PCU - Urea 0.972 PCU – Urea 0.774 PCU - Urea 0.712

Fine Branches Previous Growth Ring Organic Horizon Control – CUF 0.081 Control – CUF 0.770 Control – CUF 0.350 Control – NBPT 0.065 Control – NBPT 0.035 Control – NBPT 0.150 Control – PCU 0.294 Control – PCU 0.133 Control – PCU 0.155 Control - Urea 0.239 Control - Urea 0.247 Control - Urea 0.717 CUF - NBPT 1.000 CUF - NBPT 0.303 CUF - NBPT 0.984 CUF - PCU 0.946 CUF – PCU 0.681 CUF - PCU 0.986 CUF - Urea 0.973 CUF – Urea 0.869 CUF - Urea 0.967






Control - Urea 0.007 Control - Urea 0.771 Control - Urea 0.635 CUF - NBPT 0.917 CUF - NBPT 0.966 CUF - NBPT 0.990 CUF - PCU 0.992 CUF - PCU 0.880 CUF - PCU 0.802 CUF - Urea 0.945 CUF - Urea 0.925 CUF - Urea 0.234



Bark Coarse Roots (> 2 mm) 15-30 cm Mineral Soil Control – CUF 0.891 Control – CUF 0.015 Control – CUF 0.371 Control – NBPT 0.192 Control – NBPT 0.052 Control – NBPT 0.717 Control – PCU 0.091 Control – PCU 0.588 Control – PCU 0.322 Control - Urea 0.668 Control - Urea 0.614 Control - Urea 0.745 CUF - NBPT 0.651 CUF - NBPT 0.974 CUF - NBPT 0.974 CUF - PCU 0.410 CUF - PCU 0.263 CUF - PCU 1.000 CUF - Urea 0.992 CUF - Urea 0.245 CUF - Urea 0.966



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individual component fertilizer N recovery (% of fertilizer N applied) for Study 2 (spring 2012)


was 224 kg N ha-1. N =12.

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Differences in

Fertilizer N

Treatments

Adjusted

P-Value

Foliage Current Growth Ring Litterfall Control – CUF 0.000 Control – CUF 0.001 Control – CUF 0.017




CUF - NBPT 0.165 CUF - NBPT 0.402 CUF - NBPT 0.996 CUF - PCU 0.638 CUF - PCU 0.870 CUF - PCU 0.984

CUF - Urea 1.000 CUF - Urea 0.963 CUF - Urea 0.997 NBPT - PCU 0.005 NBPT - PCU 0.062 NBPT - PCU 0.898 NBPT - Urea 0.237 NBPT - Urea 0.122 NBPT - Urea 0.956 PCU - Urea 0.518 PCU - Urea 0.998 PCU - Urea 1.000

Fine Branches Previous Growth Ring Organic Horizon Control – CUF 0.000 Control – CUF 0.000 Control – CUF 0.191











CUF - NBPT 1.000 CUF - NBPT 0.589 CUF - NBPT 1.000 CUF - PCU 0.994 CUF - PCU 0.661 CUF - PCU 0.999

CUF - Urea 0.995 CUF - Urea 0.001 CUF - Urea 0.521 NBPT - PCU 0.986 NBPT - PCU 1.000 NBPT - PCU 0.995 NBPT - Urea 0.998 NBPT - Urea 0.083 NBPT - Urea 0.625 PCU - Urea 0.925 PCU - Urea 0.063 PCU - Urea 0.384








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Table A.4.8. The mean 15N (‰), N (g kg-1) and fertilizer N recovery (% of fertilizer N applied)

for individual ecosystem components for loblolly pine stands in the southern United States

selected to evaluate ecosystem partitioning of fertilizer N for urea or enhanced efficiency N

containing fertilizers enriched with 15N. Data is for Study 1 (spring 2011, summer 2011).

Different letters represent significant differences at α = 0.05. Numbers in parentheses represent

the standard error of the mean. The N application rate was 224 kg N ha-1. N = 5.

Spring (n=5) Summer (n=5) Ecosystem

Component Treatment

15N

(‰)

N

(g kg-1)

Fertilizer N

Recovery

15N

(‰)

N

(g kg-1)

Fertilizer N

Recovery

Foliage

Control -3.47a (0.96) 12.4a (0.3) N/A -2.98a (0.38) 12.3a (0.6) N/A CUF 106.4b (8.2) 14.2a (0.6) 13.6a (2.7) 86.7b (17.9) 14.1a (0.7) 9.8a (3.4)

NBPT 110.1b (14.3) 15.0a (1.2) 17.3a (4.0) 84.2b (9.2) 15.1a (0.9) 12.3a (4.2) PCU 95.1b (15.7) 14.9a (0.4) 9.8a (1.2) 72.7b (9.0) 14.0a (0.5) 8.4a (1.5) Urea 105.4b (10.1) 14.7a (0.5) 13.6a (4.4) 80.0b (11.7) 14.7a (0.1) 10.7a (2.5)

Fine Branches

Control -3.96a (1.17) 4.8a (0.2) N/A -2.13a (0.91) 5.1a (0.3) N/A CUF 93.7b (4.5) 7.2ab (0.5) 2.0a (0.5) 80.2b (14.3) 7.5b (0.6) 1.4a (0.4)

NBPT 88.8b (9.6) 7.5b (0.8) 2.2a (0.6) 77.9b (9.6) 7.2b (0.5) 1.5a (0.6) PCU 82.5b (13.1) 7.2ab (0.3) 1.1a (0.3) 65.9b (5.3) 6.6ab (0.4) 1.1a (0.2) Urea 83.4b (6.7) 6.8ab (0.6) 1.3a (0.4) 66.8b (16.5) 6.4ab (0.6) 1.1b (0.4)

Coarse Branches

Control -2.38a (0.98) 3.7a (0.6) N/A -2.82a (0.71) 3.0a (0.5) N/A CUF 69.2b (6.5) 3.4a (0.7) 5.7ab (1.1) 47.7b (6.8) 3.3a (0.5) 3.5a (0.9)

NBPT 68.4b (9.0) 3.3a (0.5) 7.3a (1.0) 51.3b (7.7) 3.3a (0.3) 2.6a (0.7) PCU 52.0b (7.9) 3.1a (0.3) 2.9b (0.4) 53.6b (9.3) 3.5a (0.7) 4.0a (1.1) Urea 54.0b (5.2) 3.2a (0.4) 3.9b (0.8) 57.4b (3.8) 3.2a (0.3) 4.3a (0.8)

Bark

Control -2.54a (1.03) 2.3a (0.3) N/A -2.99a (1.87) 2.1a (0.2) N/A CUF 20.2b (5.9) 2.1a (0.3) 1.3a (0.7) 10.3ab (2.5) 2.1a (0.2) 0.4a (0.2)

NBPT 19.0b (6.0) 2.7a (0.5) 1.1a (0.3) 18.8b (7.9) 2.2a (0.3) 1.0a (0.4) PCU 17.3ab (5.6) 2.5a (0.1) 1.0a (0.3) 14.7b (1.4) 3.4a (0.9) 1.2a (0.5) Urea 14.6ab (3.6) 2.4a (0.2) 0.7a (0.1) 15.4b (3.3) 2.3a (0.2) 0.6a (0.1)

Growth Ring (CGR) year of

sampling



Growth Rings (PGR) composited prior to year of sampling

Control -3.21a (1.78) 0.9a (0.2) N/A -3.55a (1.61) 1.0 a (0.2) N/A CUF 44.0b (4.9) 1.2a (0.2) 4.9a (0.8) 27.3b (3.6) 1.1a (0.3) 1.9a (0.7)


Litterfall (Current

Year)

Control -2.23a (0.93) 8.3a (0.2) N/A -2.59a (0.90) 7.8a (0.8) N/A CUF 36.4a (14.1) 6.9a (0.7) 0.5a (0.2) 23.0a (3.05) 5.4a (0.9) 0.26a (0.1)

NBPT 33.6a (6.5) 5.7a (1.2) 0.5a (0.2) 28.4a (14.6) 6.1a (1.4) 0.4a (0.2) PCU 40.2a (12.9) 6.6a (1.1) 0.6a (0.4) 27.4a (11.1) 5.5a (0.5) 0.4a (0.2) Urea 33.9a (10.3) 6.0a (0.7) 0.4a (0.2) 23.9a (9.8) 4.2a (1.0) 0.1a (0.1)

Fine Roots (< 2 mm)

Control -0.10a (0.19) 11.7a (0.7) N/A 3.1a (2.1) 13.0a (0.9) N/A CUF 21.5b (7.0) 12.8a (0.9) 8.8a (2.8) 26.7b (2.1) 11.7a (0.9) 13.4a (1.9)

NBPT 24.0b (4.3) 12.8a (0.9) 14.7a (3.8) 15.6ab (4.9) 13.5a (1.1) 10.5a (3.0) PCU 24.2b (5.5) 12.5a (1.0) 12.6a (1.8) 14.4ab (1.8) 12.2a (0.6) 9.2a (1.8) Urea 14.0ab (2.3) 12.2a (1.1) 10.2a (2.1) 14.1ab (2.2) 12.2a (0.7) 9.8a (3.3)

J.E. Raymond

218

Spring (n=5) Summer (n=5) Ecosystem

Component Treatment

15N

(‰)

N

(g kg-1)

Fertilizer N

Recovery

15N

(‰)

N

(g kg-1)

Fertilizer N

Recovery

Coarse Roots

(> 2mm)

Control 0.75a (0.72) 9.1a (0.9) N/A 1.0a (0.8) 8.7a (0.5) N/A CUF 16.3b (4.3) 8.8a (0.7) 2.3a (0.9) 15.2c (2.4) 7.1a (0.5) 1.8a (0.6)

NBPT 15.2b (2.2) 7.0a (0.2) 3. 3a (1.5) 12.9bc (2.0) 8.4a (0.8) 1.5a (0.3) PCU 16.3b (2.2) 7.0a (0.3) 2. 4a (0.9) 8.2b (0.9) 6.9a (0.3) 0.7a (0.3) Urea 8.4ab (1.9) 6.9a (0.5) 1.0a (0.4) 8.6b (0.8) 6.7a (0.5) 0.7a (0.2)

Organic Horizon




Control 3.86a (0.76) 0.8a (0.1) N/A 3.01a (0.53) 1.2a (0.2) N/A CUF 17.6b (3.3) 1.1a (0.3) 28.5b (6.6) 15.6b (2.3) 1.3a (0.3) 27.2a (4.7)

NBPT 14.4b (1.7) 1.1a (0.3) 22.4ab (3.5) 17.7b (2.8) 1.1a (0.3) 30.7a (7.6)

PCU 16.4b (3.8) 1.4a (0.4) 26.8ab (4.6) 19.7b (4.3) 1.2a (0.4) 35.7a (6.9)

Urea 10.8b (1.9) 1.1a (0.4) 16.1a (6.1) 9.60b (2.4) 1.4a (0.5) 10.7b (4.4)


Control 5.12a (0.37) 0.5a (0.2) N/A 5.80a (0.7) 0.7a (0.2) N/A CUF 11.0b (1.8) 0.7a (0.1) 6.5a (2.3) 12.7a (4.5) 0.8a (0.2) 10.0a (6.6)

NBPT 9.7ab (1.1) 0.8a (0.4) 5.5a (1.7) 10.8a (1.81) 0.7a (0.2) 6.8a (2.1) PCU 9.7ab (1.3) 0.8a (0.3) 5.2a (1.3) 12.7a (2.6) 0.9a (0.4) 10.6a (2.9) Urea 8.6ab (1.1) 0.7a (0.2) 3.6a (1.1) 8.3a (0.7) 1.1a (0.6) 6.5a (4.1)

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